diff --git a/mods/diff/hooks/ask/armed-ask/armed-ask.ts b/mods/diff/hooks/ask/armed-ask/armed-ask.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e760eb157e496793802fe46c1256e64cee0d3b03
--- /dev/null
+++ b/mods/diff/hooks/ask/armed-ask/armed-ask.ts
@@ -0,0 +1,9 @@
+/**
+ * The file the person asked about from the pane: its path and its diff as
+ * text, held until the next prompt carries it as context.
+ */
+export type ArmedAsk = {
+  path: string
+  text: string
+  lines: number
+}
diff --git a/mods/diff/hooks/ask/armed-ask/index.ts b/mods/diff/hooks/ask/armed-ask/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c6efab893c558c1b9dea831f76ca2377e2458c50
--- /dev/null
+++ b/mods/diff/hooks/ask/armed-ask/index.ts
@@ -0,0 +1,3 @@
+export type * from './armed-ask.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/ask/ask-cut-note/ask-cut-note.ts b/mods/diff/hooks/ask/ask-cut-note/ask-cut-note.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f4b1edbdca85399e4615ce753419f0dafc864cce
--- /dev/null
+++ b/mods/diff/hooks/ask/ask-cut-note/ask-cut-note.ts
@@ -0,0 +1,5 @@
+/**
+ * The line an ask ends with when its diff was cut to fit the prompt.
+ */
+export const ASK_CUT_NOTE =
+  '(The rest of this diff was cut: it did not fit in the prompt.)'
diff --git a/mods/diff/hooks/ask/ask-cut-note/index.ts b/mods/diff/hooks/ask/ask-cut-note/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f18978fc434c01c53d559b5c780b0a1c6d4b2f22
--- /dev/null
+++ b/mods/diff/hooks/ask/ask-cut-note/index.ts
@@ -0,0 +1,3 @@
+export * from './ask-cut-note.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/backend/types/backend-host/backend-host.ts b/mods/diff/hooks/backend/types/backend-host/backend-host.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0b2bfe3cf4a0b027846a32487b87fe16aae69878
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-host/backend-host.ts
@@ -0,0 +1,23 @@
+import type Git from '../../../git'
+import type { Host } from '../../../host'
+
+/**
+ * What pinning a backend takes from the bound host: its raw process runner,
+ * file probes, clock, session start and branch-base listener.
+ *
+ * Each backend names its own program, argv lead, environment and timeout
+ * to the runner; the session's start is read as it stands at each fetch
+ * (a later `session.start` moves it).
+ */
+export type BackendHost = Pick<Host, 'run' | 'readFile'> &
+  Pick<Git.GitDeps, 'mtimeOf' | 'entryKindsOf' | 'onBranchBase'> & {
+    /**
+     * The engine's clock, in milliseconds (`$.clock.now`).
+     */
+    nowMs: () => Promise<number>
+
+    /**
+     * When the current session began, read at each fetch.
+     */
+    sessionStartMsOf: () => number
+  }
diff --git a/mods/diff/hooks/backend/types/backend-host/index.ts b/mods/diff/hooks/backend/types/backend-host/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..cd04819f2cc4a6f2ed497266431266d3d4b72724
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-host/index.ts
@@ -0,0 +1,3 @@
+export type * from './backend-host.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/backend/types/backend-probe/backend-probe.ts b/mods/diff/hooks/backend/types/backend-probe/backend-probe.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d90827c7483cb18f626b67ccca7b640cf4c22e7a
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-probe/backend-probe.ts
@@ -0,0 +1,10 @@
+import type { Backend } from '../backend'
+import type { BackendHost } from '../backend-host'
+
+/**
+ * Probes the session's directory for one backend's working tree: the
+ * backend when it holds the directory, else null.
+ *
+ * Null too where git should govern it instead (Git.gitBackendOf is git's).
+ */
+export type BackendProbe = (host: BackendHost) => Promise<Backend | null>
diff --git a/mods/diff/hooks/backend/types/backend-probe/index.ts b/mods/diff/hooks/backend/types/backend-probe/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..47f881811320b8a6da746e168ca77fa494f3ec2b
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-probe/index.ts
@@ -0,0 +1,3 @@
+export type * from './backend-probe.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/backend/types/backend-words/backend-words.ts b/mods/diff/hooks/backend/types/backend-words/backend-words.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e1b0a31840bb51932534550670e6234ed5109c3f
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-words/backend-words.ts
@@ -0,0 +1,29 @@
+/**
+ * The words the pane draws that name the backend.
+ *
+ * What uncommitted mode compares against, the command whose failure the
+ * empty state names, the tool that lists untracked files, and the note
+ * under an untracked row.
+ */
+export type BackendWords = {
+  /**
+   * What the working tree is compared with outside branch mode: `HEAD`.
+   */
+  base: string
+
+  /**
+   * The command a failed fetch could not read: `git diff`.
+   */
+  diffCommand: string
+
+  /**
+   * The program that lists untracked files: `git`.
+   */
+  lister: string
+
+  /**
+   * The dim lines under an untracked row: what it is, and how to get its
+   * line counts (a pasteable command only when the path is safe to paste).
+   */
+  untrackedNoteOf: (path: string, isPasteable: boolean) => readonly string[]
+}
diff --git a/mods/diff/hooks/backend/types/backend-words/index.ts b/mods/diff/hooks/backend/types/backend-words/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8fc8e52711586492e8bf7c9af9ed8869d6be5d61
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend-words/index.ts
@@ -0,0 +1,3 @@
+export type * from './backend-words.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/backend/types/backend/backend.ts b/mods/diff/hooks/backend/types/backend/backend.ts
new file mode 100644
index 0000000000000000000000000000000000000000..53486342d24d687aff84055e82c052f01ee5a490
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend/backend.ts
@@ -0,0 +1,45 @@
+import type Git from '../../../git'
+import type { BackendWords } from '../backend-words'
+
+/**
+ * The version-control system behind the pane, pinned once per session: its
+ * working tree, what it compares, how it words them, and the pane's reads.
+ *
+ * Git is the plugin's own backend; a host may install probes for others,
+ * consulted where no git working tree holds the directory (backendOf).
+ */
+export type Backend = {
+  /**
+   * The working tree's top; every row's path is relative to it.
+   */
+  repository: Pick<Git.Repository, 'toplevel'>
+
+  /**
+   * What the picker offers to compare against, in its order.
+   */
+  baseModes: readonly Git.BaseMode[]
+
+  /**
+   * The pane's words for what this backend compares and runs.
+   */
+  words: BackendWords
+
+  /**
+   * One fetch of the working tree against the mode's base (Git.fetchDiff).
+   */
+  fetchDiff: (mode: Git.BaseMode) => Promise<Git.FetchOutcome>
+
+  /**
+   * One row's hunks against the base its fetch read (Git.fetchFileHunks).
+   */
+  fetchFileHunks: (
+    data: Git.DiffData,
+    file: Git.FileStat,
+  ) => Promise<Git.FileHunks | null>
+
+  /**
+   * A string that changes when the checked-out commit moves
+   * (Git.headKeyOf); empty when it cannot be read this tick.
+   */
+  headKeyOf: () => Promise<string>
+}
diff --git a/mods/diff/hooks/backend/types/backend/index.ts b/mods/diff/hooks/backend/types/backend/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7c3a5c071d40e4ca97948aa3daeb590c0c140282
--- /dev/null
+++ b/mods/diff/hooks/backend/types/backend/index.ts
@@ -0,0 +1,3 @@
+export type * from './backend.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/backend/types/index.ts b/mods/diff/hooks/backend/types/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..a4e249ce2343d53301111ea3b36b6171fbf293d3
--- /dev/null
+++ b/mods/diff/hooks/backend/types/index.ts
@@ -0,0 +1,6 @@
+export * from './backend'
+export * from './backend-host'
+export * from './backend-probe'
+export * from './backend-words'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/base-name-of/base-name-of.ts b/mods/diff/hooks/classify/base-name-of/base-name-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..72a503911c69f800fb813fdca2811216991dacba
--- /dev/null
+++ b/mods/diff/hooks/classify/base-name-of/base-name-of.ts
@@ -0,0 +1,7 @@
+/**
+ * A `/`-separated path's last segment: the file's own name.
+ *
+ * @param path the repository-relative path
+ * @returns the base name
+ */
+export const baseNameOf = (path: string) => path.split('/').at(-1) ?? path
diff --git a/mods/diff/hooks/classify/base-name-of/index.ts b/mods/diff/hooks/classify/base-name-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..af27a138cdcfbdbee52e00810e4dedea2310c5b9
--- /dev/null
+++ b/mods/diff/hooks/classify/base-name-of/index.ts
@@ -0,0 +1,3 @@
+export * from './base-name-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-directories/generated-directories.ts b/mods/diff/hooks/classify/is-generated-file/generated-directories/generated-directories.ts
new file mode 100644
index 0000000000000000000000000000000000000000..49e762812e8322739f150f2b27ff5095334d3795
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-directories/generated-directories.ts
@@ -0,0 +1,28 @@
+/**
+ * Path segments of build output, vendored code, caches and snapshots, the
+ * engine's excluded directories, matched against `/<path>`.
+ */
+export const GENERATED_DIRECTORIES = [
+  '/dist/',
+  '/build/',
+  '/out/',
+  '/output/',
+  '/node_modules/',
+  '/vendor/',
+  '/vendored/',
+  '/third_party/',
+  '/third-party/',
+  '/external/',
+  '/.next/',
+  '/.nuxt/',
+  '/.svelte-kit/',
+  '/coverage/',
+  '/__pycache__/',
+  '/.tox/',
+  '/venv/',
+  '/.venv/',
+  '/target/release/',
+  '/target/debug/',
+  '.generated/',
+  '/__snapshots__/',
+] as const
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-directories/index.ts b/mods/diff/hooks/classify/is-generated-file/generated-directories/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..13a8df21511ca02a107d8d70d3481e4a04c183c5
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-directories/index.ts
@@ -0,0 +1,3 @@
+export * from './generated-directories.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/generated-filename-patterns.ts b/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/generated-filename-patterns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f2d9fc814fc99efe773860689b4ca0e4a1de5c9d
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/generated-filename-patterns.ts
@@ -0,0 +1,21 @@
+/**
+ * File-name shapes of generated code, the engine's excluded file-name
+ * patterns: minified, bundled, stubs, specs, snapshots.
+ */
+export const GENERATED_FILENAME_PATTERNS: readonly RegExp[] = Object.freeze([
+  /\.min\.[a-z]+$/i,
+  /-min\.[a-z]+$/i,
+  /\.bundle\.[a-z]+$/i,
+  /\.generated\.[a-z]+$/i,
+  /\.gen\.[a-z]+$/i,
+  /\.auto\.[a-z]+$/i,
+  /_generated\.[a-z]+$/i,
+  /_gen\.[a-z]+$/i,
+  /\.pb\.(?:go|js|ts|py|rb)$/i,
+  /_pb2?\.py$/i,
+  /\.pb\.h$/i,
+  /\.grpc\.[a-z]+$/i,
+  /\.swagger\.[a-z]+$/i,
+  /\.openapi\.[a-z]+$/i,
+  /\.snap$/i,
+])
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/index.ts b/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c01ca1456171420abc51f8268d5a9e33af912998
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-filename-patterns/index.ts
@@ -0,0 +1,3 @@
+export * from './generated-filename-patterns.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-filenames/generated-filenames.ts b/mods/diff/hooks/classify/is-generated-file/generated-filenames/generated-filenames.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8b152a35dabb3dc4d35a9413435fde1ba3e789b4
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-filenames/generated-filenames.ts
@@ -0,0 +1,18 @@
+/**
+ * File names (lowercase) that are lockfiles or shrinkwraps, never hand
+ * written, so their churn is noise in a diff.
+ */
+export const GENERATED_FILENAMES: ReadonlySet<string> = new Set([
+  'package-lock.json',
+  'yarn.lock',
+  'pnpm-lock.yaml',
+  'bun.lockb',
+  'bun.lock',
+  'composer.lock',
+  'gemfile.lock',
+  'cargo.lock',
+  'poetry.lock',
+  'pipfile.lock',
+  'shrinkwrap.json',
+  'npm-shrinkwrap.json',
+])
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-filenames/index.ts b/mods/diff/hooks/classify/is-generated-file/generated-filenames/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..84bf2e29bc9f04a71fc1f27eac8073067113f2af
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-filenames/index.ts
@@ -0,0 +1,3 @@
+export * from './generated-filenames.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-suffixes/generated-suffixes.ts b/mods/diff/hooks/classify/is-generated-file/generated-suffixes/generated-suffixes.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7a530982ac63e0114efde5ff7aa3a6f1267e7f44
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-suffixes/generated-suffixes.ts
@@ -0,0 +1,15 @@
+/**
+ * Extensions, simple and compound (lowercase), of minified, bundled or
+ * generated output, the engine's excluded extensions.
+ */
+export const GENERATED_SUFFIXES = [
+  '.lock',
+  '.min.js',
+  '.min.css',
+  '.min.html',
+  '.bundle.js',
+  '.bundle.css',
+  '.generated.ts',
+  '.generated.js',
+  '.d.ts',
+] as const
diff --git a/mods/diff/hooks/classify/is-generated-file/generated-suffixes/index.ts b/mods/diff/hooks/classify/is-generated-file/generated-suffixes/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c53be33e94063642eb08dd200e6a54c8885f298a
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/generated-suffixes/index.ts
@@ -0,0 +1,3 @@
+export * from './generated-suffixes.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/index.ts b/mods/diff/hooks/classify/is-generated-file/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..13d013089881971021d49dfb9313897c879292fb
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/index.ts
@@ -0,0 +1,7 @@
+export * from './generated-directories'
+export * from './generated-filename-patterns'
+export * from './generated-filenames'
+export * from './generated-suffixes'
+export * from './is-generated-file.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-generated-file/is-generated-file.ts b/mods/diff/hooks/classify/is-generated-file/is-generated-file.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7ced1855f65b9d3426c09613f19700b3c0b0b532
--- /dev/null
+++ b/mods/diff/hooks/classify/is-generated-file/is-generated-file.ts
@@ -0,0 +1,29 @@
+import { baseNameOf } from '../base-name-of'
+import { rootedPathOf } from '../rooted-path-of'
+import { GENERATED_DIRECTORIES } from './generated-directories'
+import { GENERATED_FILENAME_PATTERNS } from './generated-filename-patterns'
+import { GENERATED_FILENAMES } from './generated-filenames'
+import { GENERATED_SUFFIXES } from './generated-suffixes'
+
+/**
+ * Whether a repository-relative path names generated or vendored content,
+ * by the Linguist-style rules the engine's own diff panel applies.
+ *
+ * @param path the file's path from the repository root, `/`-separated
+ * @returns whether the diff pane counts it among the tests and generated files
+ */
+export function isGeneratedFile(path: string) {
+  const name = baseNameOf(path).toLowerCase()
+  const rooted = rootedPathOf(path)
+
+  const hasGeneratedSuffix = GENERATED_SUFFIXES.some(suffix =>
+    name.endsWith(suffix),
+  )
+
+  return (
+    GENERATED_FILENAMES.has(name) ||
+    hasGeneratedSuffix ||
+    GENERATED_DIRECTORIES.some(directory => rooted.includes(directory)) ||
+    GENERATED_FILENAME_PATTERNS.some(pattern => pattern.test(name))
+  )
+}
diff --git a/mods/diff/hooks/classify/is-test-file/index.ts b/mods/diff/hooks/classify/is-test-file/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..def0bac974e35906140bb41f70d2eef0da980de4
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/index.ts
@@ -0,0 +1,5 @@
+export * from './is-test-file.js'
+export * from './test-directories'
+export * from './test-filename-patterns'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-test-file/is-test-file.ts b/mods/diff/hooks/classify/is-test-file/is-test-file.ts
new file mode 100644
index 0000000000000000000000000000000000000000..2cb41a128777bdaeeafd96eba5cbdd6b3fadb4fc
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/is-test-file.ts
@@ -0,0 +1,21 @@
+import { baseNameOf } from '../base-name-of'
+import { rootedPathOf } from '../rooted-path-of'
+import { TEST_DIRECTORIES } from './test-directories'
+import { TEST_FILENAME_PATTERNS } from './test-filename-patterns'
+
+/**
+ * Whether a repository-relative path is a test, spec, fixture or snapshot,
+ * by the same rules the engine's own diff panel applies.
+ *
+ * @param path the file's path from the repository root, `/`-separated
+ * @returns whether the diff pane counts it among the tests and generated files
+ */
+export function isTestFile(path: string) {
+  const name = baseNameOf(path)
+  const rooted = rootedPathOf(path)
+
+  return (
+    TEST_DIRECTORIES.some(directory => rooted.includes(directory)) ||
+    TEST_FILENAME_PATTERNS.some(pattern => pattern.test(name))
+  )
+}
diff --git a/mods/diff/hooks/classify/is-test-file/test-directories/index.ts b/mods/diff/hooks/classify/is-test-file/test-directories/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..cecf4b11d44164ed9c59a78862acf3399cb3a313
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/test-directories/index.ts
@@ -0,0 +1,3 @@
+export * from './test-directories.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-test-file/test-directories/test-directories.ts b/mods/diff/hooks/classify/is-test-file/test-directories/test-directories.ts
new file mode 100644
index 0000000000000000000000000000000000000000..111c4e2077357d56f2b16b272b92f11b6879c358
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/test-directories/test-directories.ts
@@ -0,0 +1,16 @@
+/**
+ * Path segments of tests, specs, mocks and fixtures, the engine's test
+ * directories, matched against `/<path>`.
+ */
+export const TEST_DIRECTORIES = [
+  '/test/',
+  '/tests/',
+  '/spec/',
+  '/specs/',
+  '/__tests__/',
+  '/__mocks__/',
+  '/__snapshots__/',
+  '/__fixtures__/',
+  '/fixtures/',
+  '/testdata/',
+] as const
diff --git a/mods/diff/hooks/classify/is-test-file/test-filename-patterns/index.ts b/mods/diff/hooks/classify/is-test-file/test-filename-patterns/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..56f60e7855be6f084c838509556636a70111aabf
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/test-filename-patterns/index.ts
@@ -0,0 +1,3 @@
+export * from './test-filename-patterns.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/is-test-file/test-filename-patterns/test-filename-patterns.ts b/mods/diff/hooks/classify/is-test-file/test-filename-patterns/test-filename-patterns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6c472162d1ea3ea7cea68447c948247a2131f32b
--- /dev/null
+++ b/mods/diff/hooks/classify/is-test-file/test-filename-patterns/test-filename-patterns.ts
@@ -0,0 +1,11 @@
+/**
+ * File-name shapes of test files and snapshots, the engine's test file-name
+ * patterns.
+ */
+export const TEST_FILENAME_PATTERNS: readonly RegExp[] = Object.freeze([
+  /\.test\.[a-z]+$/i,
+  /\.spec\.[a-z]+$/i,
+  /_test\.[a-z]+$/i,
+  /_spec\.[a-z]+$/i,
+  /\.snap$/i,
+])
diff --git a/mods/diff/hooks/classify/rooted-path-of/index.ts b/mods/diff/hooks/classify/rooted-path-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..827b27f32f2c45c92a6cd2c76b306369a9aff199
--- /dev/null
+++ b/mods/diff/hooks/classify/rooted-path-of/index.ts
@@ -0,0 +1,3 @@
+export * from './rooted-path-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/classify/rooted-path-of/rooted-path-of.ts b/mods/diff/hooks/classify/rooted-path-of/rooted-path-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..47411f4513080eb6cd94364bf1f773ced73dbc81
--- /dev/null
+++ b/mods/diff/hooks/classify/rooted-path-of/rooted-path-of.ts
@@ -0,0 +1,8 @@
+/**
+ * A path with exactly one leading `/`, so a directory pattern like `/test/`
+ * matches at the start of a relative path too.
+ *
+ * @param path the repository-relative path
+ * @returns the rooted path
+ */
+export const rootedPathOf = (path: string) => `/${path.replace(/^\/+/, '')}`
diff --git a/mods/diff/hooks/limits/columns/auto-open-min-columns.ts b/mods/diff/hooks/limits/columns/auto-open-min-columns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..edc538909dc36ae64af606420876007c949124b4
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/auto-open-min-columns.ts
@@ -0,0 +1,5 @@
+/**
+ * The terminal width from which the first edit opens the pane when the person
+ * never said whether they want it (the built-in sidebar's own 144).
+ */
+export const AUTO_OPEN_MIN_COLUMNS = 144
diff --git a/mods/diff/hooks/limits/columns/dialog-path-reserve.ts b/mods/diff/hooks/limits/columns/dialog-path-reserve.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ada8ca20158e1112056b40e0375576e56b375716
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/dialog-path-reserve.ts
@@ -0,0 +1,5 @@
+/**
+ * The cells an inline file row keeps off its path (DiffFileList's 16 for
+ * the counts, 3 for the pointer, 4 for the frame).
+ */
+export const DIALOG_PATH_RESERVE = 23
diff --git a/mods/diff/hooks/limits/columns/index.ts b/mods/diff/hooks/limits/columns/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6b8ddd35a7342c0cf5aaf5fcad8db92e9b2fddf0
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/index.ts
@@ -0,0 +1,9 @@
+export * from './auto-open-min-columns.js'
+export * from './dialog-path-reserve.js'
+export * from './open-min-columns.js'
+export * from './pane-right-pad-columns.js'
+export * from './path-floor.js'
+export * from './stat-cells.js'
+export * from './widest-bucket-columns.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/limits/columns/open-min-columns.ts b/mods/diff/hooks/limits/columns/open-min-columns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3245b76307420e1de6c7c4208f7aa24fa1e15329
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/open-min-columns.ts
@@ -0,0 +1,8 @@
+/**
+ * The terminal width from which `/diff` opens the pane (the built-in
+ * sidebar's own minimum).
+ *
+ * The first edit opens it from this width too, when the person kept it
+ * open before.
+ */
+export const OPEN_MIN_COLUMNS = 110
diff --git a/mods/diff/hooks/limits/columns/pane-right-pad-columns.ts b/mods/diff/hooks/limits/columns/pane-right-pad-columns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9dfef13a5700ee40137a29f221f71ad14422cd9f
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/pane-right-pad-columns.ts
@@ -0,0 +1,5 @@
+/**
+ * The blank column the docked pane keeps at its right edge, as the built-in
+ * sidebar's body does (its `paddingX`): rows wrap one column short of it.
+ */
+export const PANE_RIGHT_PAD_COLUMNS = 1
diff --git a/mods/diff/hooks/limits/columns/path-floor.ts b/mods/diff/hooks/limits/columns/path-floor.ts
new file mode 100644
index 0000000000000000000000000000000000000000..630017b6bcf22044f79dea00a860f405f0a6260f
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/path-floor.ts
@@ -0,0 +1,5 @@
+/**
+ * The fewest cells a docked file row gives its path, however narrow the
+ * pane (ReplDiffSidebar's 8).
+ */
+export const PATH_FLOOR = 8
diff --git a/mods/diff/hooks/limits/columns/stat-cells.ts b/mods/diff/hooks/limits/columns/stat-cells.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4bc6692b051b8b003142716b0cd6bf87b97d7ca4
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/stat-cells.ts
@@ -0,0 +1,5 @@
+/**
+ * The cells a docked file row keeps for its `+N -M` (ReplDiffSidebar's 12):
+ * the path is cut from its start to the rest.
+ */
+export const STAT_CELLS = 12
diff --git a/mods/diff/hooks/limits/columns/widest-bucket-columns.ts b/mods/diff/hooks/limits/columns/widest-bucket-columns.ts
new file mode 100644
index 0000000000000000000000000000000000000000..cb4d66860e7b3d5484055d1f0d3c93d65443f711
--- /dev/null
+++ b/mods/diff/hooks/limits/columns/widest-bucket-columns.ts
@@ -0,0 +1,4 @@
+/**
+ * The terminal width from which `diff_pane_shown` reports `200_plus`.
+ */
+export const WIDEST_BUCKET_COLUMNS = 200
diff --git a/mods/diff/hooks/limits/sizes/bytes/host-output-cap-bytes.ts b/mods/diff/hooks/limits/sizes/bytes/host-output-cap-bytes.ts
new file mode 100644
index 0000000000000000000000000000000000000000..08128af0b373bd7ce8e72dc45c90b69d22d1d673
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/bytes/host-output-cap-bytes.ts
@@ -0,0 +1,5 @@
+/**
+ * The most of a child's stdout `$.process.run` hands back, in bytes
+ * (process-host OUTPUT_LIMIT); a listing that long was cut, silently.
+ */
+export const HOST_OUTPUT_CAP_BYTES = 4 * 1024 * 1024
diff --git a/mods/diff/hooks/limits/sizes/bytes/index.ts b/mods/diff/hooks/limits/sizes/bytes/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d0f573676e347249fb24b807cad36a7400387222
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/bytes/index.ts
@@ -0,0 +1,4 @@
+export * from './host-output-cap-bytes.js'
+export * from './max-diff-bytes.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/limits/sizes/bytes/max-diff-bytes.ts b/mods/diff/hooks/limits/sizes/bytes/max-diff-bytes.ts
new file mode 100644
index 0000000000000000000000000000000000000000..18be71cea03ffb023e5c383f7622d8263e49a8d5
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/bytes/max-diff-bytes.ts
@@ -0,0 +1,5 @@
+/**
+ * A file whose unified diff text is longer than this is a "large file": its
+ * hunks are not parsed (the built-in panel's size cap).
+ */
+export const MAX_DIFF_BYTES = 1_000_000
diff --git a/mods/diff/hooks/limits/sizes/index.ts b/mods/diff/hooks/limits/sizes/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..de209816b5ecee14a1da860f0a8217159d08de89
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/index.ts
@@ -0,0 +1,11 @@
+export * from './bytes'
+export * from './max-files.js'
+export * from './max-files-for-details.js'
+export * from './max-lines-per-file.js'
+export * from './max-listed-directories.js'
+export * from './max-untracked-probes.js'
+export * from './max-visible-files.js'
+export * from './pane'
+export * from './prompt-context-max-chars.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/limits/sizes/max-files-for-details.ts b/mods/diff/hooks/limits/sizes/max-files-for-details.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7d37404f5a2b174cadbbbfd5efdd08734c2d87fd
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-files-for-details.ts
@@ -0,0 +1,5 @@
+/**
+ * Past this many changed files the shortstat totals alone are kept: no
+ * per-file rows, no hunks, no untracked probe (the built-in panel's cap).
+ */
+export const MAX_FILES_FOR_DETAILS = 500
diff --git a/mods/diff/hooks/limits/sizes/max-files.ts b/mods/diff/hooks/limits/sizes/max-files.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b1490c5c751df50f699edd67309072f514319d7f
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-files.ts
@@ -0,0 +1,5 @@
+/**
+ * How many per-file rows a diff keeps (the built-in panel's cap); the rest
+ * are counted as "not shown".
+ */
+export const MAX_FILES = 50
diff --git a/mods/diff/hooks/limits/sizes/max-lines-per-file.ts b/mods/diff/hooks/limits/sizes/max-lines-per-file.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0b953d438b4d928cda990367a61d34be334a8f0f
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-lines-per-file.ts
@@ -0,0 +1,5 @@
+/**
+ * How many diff lines of one file are kept before the body is cut and the
+ * file reads as truncated (GitHub's auto-load limit, as the built-in's).
+ */
+export const MAX_LINES_PER_FILE = 400
diff --git a/mods/diff/hooks/limits/sizes/max-listed-directories.ts b/mods/diff/hooks/limits/sizes/max-listed-directories.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4c3646a1dcd6989f4674944664202e867f15f88d
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-listed-directories.ts
@@ -0,0 +1,9 @@
+/**
+ * How many directories one fetch may list while dating its files; past it
+ * an untracked file reads as pre-session, a tracked one as session work.
+ *
+ * A walk lists only the distinct ancestors of the paths it dates (MAX_FILES
+ * tracked, MAX_UNTRACKED_PROBES untracked), so the budget in effect is the
+ * dated set's own directory count; this caps a pathological spread.
+ */
+export const MAX_LISTED_DIRECTORIES = 512
diff --git a/mods/diff/hooks/limits/sizes/max-untracked-probes.ts b/mods/diff/hooks/limits/sizes/max-untracked-probes.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8c35009e187bc2bb8eab250b6c84f2a0e2ca4aad
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-untracked-probes.ts
@@ -0,0 +1,5 @@
+/**
+ * How many untracked paths get a timestamp probe per fetch; the rest read as
+ * pre-session, since only MAX_FILES rows could ever show.
+ */
+export const MAX_UNTRACKED_PROBES = 500
diff --git a/mods/diff/hooks/limits/sizes/max-visible-files.ts b/mods/diff/hooks/limits/sizes/max-visible-files.ts
new file mode 100644
index 0000000000000000000000000000000000000000..944dbba3aac28ec555023dedc7eb8541d0715f6f
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/max-visible-files.ts
@@ -0,0 +1,5 @@
+/**
+ * The file rows the dialog lists at once (DiffFileList's MAX_VISIBLE_FILES);
+ * past them its window moves, `more files` rows counting the rest.
+ */
+export const MAX_VISIBLE_FILES = 5
diff --git a/mods/diff/hooks/limits/sizes/pane/body-fetch-concurrency.ts b/mods/diff/hooks/limits/sizes/pane/body-fetch-concurrency.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fe43ae424f8185a00b8b7c4af36017bf9a2a1136
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/body-fetch-concurrency.ts
@@ -0,0 +1,5 @@
+/**
+ * How many files' hunks are read from the backend at once while the pane
+ * loads its bodies.
+ */
+export const BODY_FETCH_CONCURRENCY = 6
diff --git a/mods/diff/hooks/limits/sizes/pane/gutter-chrome.ts b/mods/diff/hooks/limits/sizes/pane/gutter-chrome.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c0cd40c0a442a4e95c287e49c1e48628ddb30d34
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/gutter-chrome.ts
@@ -0,0 +1,5 @@
+/**
+ * The cells a diff gutter takes besides its digits: the marker and the
+ * spaces round the number.
+ */
+export const GUTTER_CHROME = 3
diff --git a/mods/diff/hooks/limits/sizes/pane/index.ts b/mods/diff/hooks/limits/sizes/pane/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6a3e2b7a5d982290ce32da504d1b0fe41dfbaba6
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/index.ts
@@ -0,0 +1,9 @@
+export * from './body-fetch-concurrency.js'
+export * from './gutter-chrome.js'
+export * from './max-summary-rows.js'
+export * from './pane-top-pad-rows.js'
+export * from './pre-session-body-cap.js'
+export * from './scroll-margin-rows.js'
+export * from './wheel-rows.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/limits/sizes/pane/max-summary-rows.ts b/mods/diff/hooks/limits/sizes/pane/max-summary-rows.ts
new file mode 100644
index 0000000000000000000000000000000000000000..bc4547618f7dc1f1676a7ab5a4bfd9348c4b2805
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/max-summary-rows.ts
@@ -0,0 +1,5 @@
+/**
+ * How many file rows the docked pane's list shows at once (the built-in
+ * panel's window); the rest are counted on a dim row under them.
+ */
+export const MAX_SUMMARY_ROWS = 8
diff --git a/mods/diff/hooks/limits/sizes/pane/pane-top-pad-rows.ts b/mods/diff/hooks/limits/sizes/pane/pane-top-pad-rows.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e487c31804c8ff48cafed19d3c5d2a498ccbe18e
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/pane-top-pad-rows.ts
@@ -0,0 +1,5 @@
+/**
+ * The blank row the docked pane keeps above its header, as the built-in
+ * sidebar's header does (its `paddingY`).
+ */
+export const PANE_TOP_PAD_ROWS = 1
diff --git a/mods/diff/hooks/limits/sizes/pane/pre-session-body-cap.ts b/mods/diff/hooks/limits/sizes/pane/pre-session-body-cap.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e46db0d58083a94869bf96af66dc3d0498e2d8cf
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/pre-session-body-cap.ts
@@ -0,0 +1,5 @@
+/**
+ * How many pre-session files get their bodies drawn once that section is
+ * open (the built-in panel's cap); past it only their legend is listed.
+ */
+export const PRE_SESSION_BODY_CAP = 20
diff --git a/mods/diff/hooks/limits/sizes/pane/scroll-margin-rows.ts b/mods/diff/hooks/limits/sizes/pane/scroll-margin-rows.ts
new file mode 100644
index 0000000000000000000000000000000000000000..15c020be3fef518e9ac545e3314f59138e0f1a53
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/scroll-margin-rows.ts
@@ -0,0 +1,5 @@
+/**
+ * The rows the docked window draws past its own height, so a line whose
+ * wrap was counted short still leaves it full; the pane clips the rest.
+ */
+export const SCROLL_MARGIN_ROWS = 8
diff --git a/mods/diff/hooks/limits/sizes/pane/wheel-rows.ts b/mods/diff/hooks/limits/sizes/pane/wheel-rows.ts
new file mode 100644
index 0000000000000000000000000000000000000000..5d0a97fb021da98129df53f974eff80dc1f028ac
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/pane/wheel-rows.ts
@@ -0,0 +1,5 @@
+/**
+ * The rows one wheel tick moves the docked body, as the built-in panel's
+ * body moves.
+ */
+export const WHEEL_ROWS = 3
diff --git a/mods/diff/hooks/limits/sizes/prompt-context-max-chars.ts b/mods/diff/hooks/limits/sizes/prompt-context-max-chars.ts
new file mode 100644
index 0000000000000000000000000000000000000000..81520f109a6912f022605c893d57b0abf4460583
--- /dev/null
+++ b/mods/diff/hooks/limits/sizes/prompt-context-max-chars.ts
@@ -0,0 +1,5 @@
+/**
+ * The most characters a prompt's context carries, its entries together; the
+ * engine refuses a context past it, so an ask is fitted to what is left.
+ */
+export const PROMPT_CONTEXT_MAX_CHARS = 32_000
diff --git a/mods/diff/hooks/limits/timing/git-timeout-ms.ts b/mods/diff/hooks/limits/timing/git-timeout-ms.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e7824e4804c57d93c173ebfbe20973e8aad65b49
--- /dev/null
+++ b/mods/diff/hooks/limits/timing/git-timeout-ms.ts
@@ -0,0 +1,5 @@
+/**
+ * How long one git child may run before the fetch gives up on it and keeps
+ * the last good diff (the built-in panel's own timeout).
+ */
+export const GIT_TIMEOUT_MS = 5000
diff --git a/mods/diff/hooks/limits/timing/head-poll-ms.ts b/mods/diff/hooks/limits/timing/head-poll-ms.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6d49c4ed71759930848a4f4b3e9d5626df7df7c2
--- /dev/null
+++ b/mods/diff/hooks/limits/timing/head-poll-ms.ts
@@ -0,0 +1,5 @@
+/**
+ * How often `.git/HEAD` and the checked-out ref are probed for a checkout or
+ * commit made outside the session.
+ */
+export const HEAD_POLL_MS = 2000
diff --git a/mods/diff/hooks/limits/timing/index.ts b/mods/diff/hooks/limits/timing/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6a9c4d91faa668bba91a85a96413bb5beda86a4c
--- /dev/null
+++ b/mods/diff/hooks/limits/timing/index.ts
@@ -0,0 +1,6 @@
+export * from './git-timeout-ms.js'
+export * from './head-poll-ms.js'
+export * from './redraw-coalesce-ms.js'
+export * from './refresh-debounce-ms.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/limits/timing/redraw-coalesce-ms.ts b/mods/diff/hooks/limits/timing/redraw-coalesce-ms.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fa46795215b1aaf23a957f12538014c01ae2b6d6
--- /dev/null
+++ b/mods/diff/hooks/limits/timing/redraw-coalesce-ms.ts
@@ -0,0 +1,5 @@
+/**
+ * How long state changes gather before one `$.ui.invalidate`, keeping the
+ * pane under the engine's ten redraws a second.
+ */
+export const REDRAW_COALESCE_MS = 100
diff --git a/mods/diff/hooks/limits/timing/refresh-debounce-ms.ts b/mods/diff/hooks/limits/timing/refresh-debounce-ms.ts
new file mode 100644
index 0000000000000000000000000000000000000000..38332e9c41711bea845b977981e5b11161ac8924
--- /dev/null
+++ b/mods/diff/hooks/limits/timing/refresh-debounce-ms.ts
@@ -0,0 +1,5 @@
+/**
+ * The trailing-edge wait between a trigger (an edit, a shell command, a turn
+ * step) and the refetch it causes, so a burst of edits fetches once.
+ */
+export const REFRESH_DEBOUNCE_MS = 150
diff --git a/mods/diff/hooks/names/keys/base-store-key-of.ts b/mods/diff/hooks/names/keys/base-store-key-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7896b6d1b9b7d6c56489fa6c191db79344c9d216
--- /dev/null
+++ b/mods/diff/hooks/names/keys/base-store-key-of.ts
@@ -0,0 +1,8 @@
+/**
+ * The `$.store` key of a working tree's chosen comparison base, one per
+ * `--show-toplevel` so a linked worktree keeps its own.
+ *
+ * @param toplevel the working tree's top-level path
+ * @returns the key
+ */
+export const baseStoreKeyOf = (toplevel: string) => `base:${toplevel}`
diff --git a/mods/diff/hooks/names/keys/index.ts b/mods/diff/hooks/names/keys/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..754b99ce989251a4f65e31868b1f70c625a99798
--- /dev/null
+++ b/mods/diff/hooks/names/keys/index.ts
@@ -0,0 +1,4 @@
+export * from './base-store-key-of.js'
+export * from './store-open-key.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/names/keys/store-open-key.ts b/mods/diff/hooks/names/keys/store-open-key.ts
new file mode 100644
index 0000000000000000000000000000000000000000..be9f3fd66a5af67613da653b92724751dd033d70
--- /dev/null
+++ b/mods/diff/hooks/names/keys/store-open-key.ts
@@ -0,0 +1,5 @@
+/**
+ * The `$.store` key holding whether the person keeps the pane open (`/diff`
+ * writes it; the first edit reads it): the built-in's `diffSidebarOpen`.
+ */
+export const STORE_OPEN_KEY = 'open'
diff --git a/mods/diff/hooks/names/texts/builtin-holds-pattern.ts b/mods/diff/hooks/names/texts/builtin-holds-pattern.ts
new file mode 100644
index 0000000000000000000000000000000000000000..afc27dc93eaacf62e996cf41087ee3f95741c479
--- /dev/null
+++ b/mods/diff/hooks/names/texts/builtin-holds-pattern.ts
@@ -0,0 +1,5 @@
+/**
+ * The one `$.command.register` refusal meaning the built-in `/diff` serves
+ * this session, as the engine words it: the plugin stands down silently.
+ */
+export const BUILTIN_HOLDS_PATTERN = /refused: it is the built-in \//
diff --git a/mods/diff/hooks/names/texts/git-unanswered-text.ts b/mods/diff/hooks/names/texts/git-unanswered-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e167dda0c823bfea222826f7bed4bf0370d2481e
--- /dev/null
+++ b/mods/diff/hooks/names/texts/git-unanswered-text.ts
@@ -0,0 +1,9 @@
+/**
+ * `/diff`'s answer when git did not say whether this is a repository.
+ *
+ * It timed out: the directory is unknown, not outside one. A git that
+ * cannot start has answered, and gets the not-a-repository text.
+ */
+export const GIT_UNANSWERED_TEXT =
+  "The diff panel couldn't read git state — git didn't answer; run /diff " +
+  'again'
diff --git a/mods/diff/hooks/names/texts/index.ts b/mods/diff/hooks/names/texts/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7e801a598375c1bb775260b4a01f958fcfe6c4dd
--- /dev/null
+++ b/mods/diff/hooks/names/texts/index.ts
@@ -0,0 +1,10 @@
+export * from './builtin-holds-pattern.js'
+export * from './git-unanswered-text.js'
+export * from './more-below-text.js'
+export * from './not-in-repository-text.js'
+export * from './panel-toggle'
+export * from './register-failed-text-of.js'
+export * from './resize-terminal-text.js'
+export * from './untracked-withheld-text-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/names/texts/more-below-text.ts b/mods/diff/hooks/names/texts/more-below-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..669d268390ce09228bad0a2218d77d2c6f8eb7fb
--- /dev/null
+++ b/mods/diff/hooks/names/texts/more-below-text.ts
@@ -0,0 +1,5 @@
+/**
+ * What follows a count on the docked list's lower edge row: the words and
+ * the built-in's advertised list key (its macOS spelling).
+ */
+export const MORE_BELOW_TEXT = 'more below (opt+\u2193 to scroll)'
diff --git a/mods/diff/hooks/names/texts/not-in-repository-text.ts b/mods/diff/hooks/names/texts/not-in-repository-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4d8283656b0a5ac6f9bc28e9424a22523c339c70
--- /dev/null
+++ b/mods/diff/hooks/names/texts/not-in-repository-text.ts
@@ -0,0 +1,7 @@
+/**
+ * `/diff`'s answer outside a git repository, worded as the built-in words it
+ * but for its apostrophe, which is ASCII.
+ */
+export const NOT_IN_REPOSITORY_TEXT =
+  "The diff panel shows git changes — the current directory isn't in a " +
+  'git repository'
diff --git a/mods/diff/hooks/names/texts/panel-toggle/dialog-dismissed-text.ts b/mods/diff/hooks/names/texts/panel-toggle/dialog-dismissed-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6f9cd8a518e93f2ddc8c3a2b06daf2c967dcb868
--- /dev/null
+++ b/mods/diff/hooks/names/texts/panel-toggle/dialog-dismissed-text.ts
@@ -0,0 +1,4 @@
+/**
+ * The transcript line the built-in dialog leaves when the person closes it.
+ */
+export const DIALOG_DISMISSED_TEXT = 'Diff dialog dismissed'
diff --git a/mods/diff/hooks/names/texts/panel-toggle/index.ts b/mods/diff/hooks/names/texts/panel-toggle/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..bf4293cb780a290c92642ea924b932e356677b91
--- /dev/null
+++ b/mods/diff/hooks/names/texts/panel-toggle/index.ts
@@ -0,0 +1,5 @@
+export * from './dialog-dismissed-text.js'
+export * from './panel-hidden-text.js'
+export * from './panel-shown-text.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/names/texts/panel-toggle/panel-hidden-text.ts b/mods/diff/hooks/names/texts/panel-toggle/panel-hidden-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..85bb37e272ed7b89902c4f2ad830f635be33690e
--- /dev/null
+++ b/mods/diff/hooks/names/texts/panel-toggle/panel-hidden-text.ts
@@ -0,0 +1,5 @@
+/**
+ * `/diff`'s transcript line once it closed the pane, worded as the built-in
+ * words its own.
+ */
+export const PANEL_HIDDEN_TEXT = 'Diff panel hidden'
diff --git a/mods/diff/hooks/names/texts/panel-toggle/panel-shown-text.ts b/mods/diff/hooks/names/texts/panel-toggle/panel-shown-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..2375f17ff29350bb92135365546e32592b366c4d
--- /dev/null
+++ b/mods/diff/hooks/names/texts/panel-toggle/panel-shown-text.ts
@@ -0,0 +1,5 @@
+/**
+ * `/diff`'s transcript line once it opened the pane, worded as the built-in
+ * words its own.
+ */
+export const PANEL_SHOWN_TEXT = 'Diff panel shown'
diff --git a/mods/diff/hooks/names/texts/register-failed-text-of.ts b/mods/diff/hooks/names/texts/register-failed-text-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..11ba7ea7e8fe52c9141b7b98454a325d693ebc6d
--- /dev/null
+++ b/mods/diff/hooks/names/texts/register-failed-text-of.ts
@@ -0,0 +1,10 @@
+/**
+ * The transcript line when `/diff` could not be registered for a reason
+ * other than the built-in holding it: the person learns the pane is off.
+ *
+ * @param reason the refusal's message
+ * @returns the line
+ */
+export const registerFailedTextOf = (reason: string) =>
+  `could not register /diff: ${reason}; the diff panel is unavailable this ` +
+  `session`
diff --git a/mods/diff/hooks/names/texts/resize-terminal-text.ts b/mods/diff/hooks/names/texts/resize-terminal-text.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4201bcc6c7c0c1dd4ceacc33da4960724e02e7d5
--- /dev/null
+++ b/mods/diff/hooks/names/texts/resize-terminal-text.ts
@@ -0,0 +1,9 @@
+/**
+ * `/diff`'s answer on a terminal narrower than the built-in panel shows
+ * on, worded as the built-in words it.
+ *
+ * Also the pane's one line where the surface seats it inline (no room
+ * beside the transcript).
+ */
+export const RESIZE_TERMINAL_TEXT =
+  'Resize your terminal to at least 110 columns to show the diff panel'
diff --git a/mods/diff/hooks/names/texts/untracked-withheld-text-of.ts b/mods/diff/hooks/names/texts/untracked-withheld-text-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8c1d5e31820a6cb2ab3bda676e69a6fdc9d64585
--- /dev/null
+++ b/mods/diff/hooks/names/texts/untracked-withheld-text-of.ts
@@ -0,0 +1,14 @@
+import type PaneState from '../../pane-state'
+
+/**
+ * The pane's word for a fetch whose untracked listing the backend did not
+ * give whole: the tracked rows stand, the new files are not counted.
+ *
+ * @param model the pane's state, its words naming the program that lists them
+ * @returns the note
+ */
+export const untrackedWithheldTextOf = (
+  model: Pick<PaneState.PaneModel, 'words'>,
+) =>
+  `Untracked files unavailable (${model.words.lister} could not list them); ` +
+  'not counted'
diff --git a/mods/diff/hooks/pane-state/after-fetch/after-fetch.ts b/mods/diff/hooks/pane-state/after-fetch/after-fetch.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8e5380d9fcb3da3d53d977b13eeb51c2f0ccabe8
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-fetch/after-fetch.ts
@@ -0,0 +1,29 @@
+import Todos from '../../todos'
+import Turns from '../../turns'
+import type { PaneModel } from '../pane-model'
+import type { Fetched } from './fetched'
+
+/**
+ * The model once a refresh has settled: the outcome's data (kept as it was
+ * when the fetch was unavailable), the transcript's turns and todos.
+ *
+ * @param model the model before the refresh
+ * @param fetched what the refresh read
+ * @returns the settled model
+ */
+export function afterFetch(model: PaneModel, fetched: Fetched): PaneModel {
+  const { outcome, messages } = fetched
+  const isData = outcome.kind === 'data'
+  const isKept = outcome.kind === 'unavailable'
+  const data = isData ? outcome.data : isKept ? model.data : null
+
+  return {
+    ...model,
+    isLoading: false,
+    hasSettled: true,
+    isOutsideRepository: outcome.kind === 'no-repository',
+    data,
+    turns: Turns.turnDiffsOf(messages),
+    todos: Todos.todoProgressOf(messages),
+  }
+}
diff --git a/mods/diff/hooks/pane-state/after-fetch/fetched/fetched.ts b/mods/diff/hooks/pane-state/after-fetch/fetched/fetched.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c93b77cdd48b0817d5401a9f5d63a22b9a64ddd0
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-fetch/fetched/fetched.ts
@@ -0,0 +1,11 @@
+import type { SessionMessage } from 'claude-code'
+
+import type Git from '../../../git'
+
+/**
+ * What one refresh read: how the diff fetch ended and the transcript rows.
+ */
+export type Fetched = {
+  outcome: Git.FetchOutcome
+  messages: readonly SessionMessage[]
+}
diff --git a/mods/diff/hooks/pane-state/after-fetch/fetched/index.ts b/mods/diff/hooks/pane-state/after-fetch/fetched/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7262baa1935f5e4cd0a19604e255609b3c8ae193
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-fetch/fetched/index.ts
@@ -0,0 +1,3 @@
+export type * from './fetched.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/after-fetch/index.ts b/mods/diff/hooks/pane-state/after-fetch/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..227de9d375672607bcca08ca76f67728589b4694
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-fetch/index.ts
@@ -0,0 +1,4 @@
+export * from './after-fetch.js'
+export * from './fetched'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/after-new-session/after-new-session.ts b/mods/diff/hooks/pane-state/after-new-session/after-new-session.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3769c1c055f46f528709429d58769c2db62bb75a
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-new-session/after-new-session.ts
@@ -0,0 +1,14 @@
+import type { PaneModel } from '../pane-model'
+import { NEW_SESSION_FIELDS } from './new-session-fields'
+
+/**
+ * The model after `/clear` or `/resume`: the fetch kept, every pick and
+ * the transcript's turns dropped.
+ *
+ * @param model the model before the switch
+ * @returns the reset model
+ */
+export const afterNewSession = (model: PaneModel): PaneModel => ({
+  ...model,
+  ...NEW_SESSION_FIELDS,
+})
diff --git a/mods/diff/hooks/pane-state/after-new-session/index.ts b/mods/diff/hooks/pane-state/after-new-session/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..04346ab5da35d41bc673778ad2623f99dd9812e0
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-new-session/index.ts
@@ -0,0 +1,4 @@
+export * from './after-new-session.js'
+export * from './new-session-fields'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/after-new-session/new-session-fields/index.ts b/mods/diff/hooks/pane-state/after-new-session/new-session-fields/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e57f39c830d34e2da8f31afd500f75f1b8543870
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-new-session/new-session-fields/index.ts
@@ -0,0 +1,3 @@
+export * from './new-session-fields.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/after-new-session/new-session-fields/new-session-fields.ts b/mods/diff/hooks/pane-state/after-new-session/new-session-fields/new-session-fields.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c7e72d7f66b196c9830928d4156795b00ed63fe5
--- /dev/null
+++ b/mods/diff/hooks/pane-state/after-new-session/new-session-fields/new-session-fields.ts
@@ -0,0 +1,12 @@
+import { NO_BODIES } from '../../no-bodies'
+
+/**
+ * What `/clear` and `/resume` reset: the pick, the source, the turns and
+ * the bodies; the fetch itself stays.
+ */
+export const NEW_SESSION_FIELDS = Object.freeze({
+  selectedPath: null,
+  source: Object.freeze({ kind: 'current' }),
+  turns: Object.freeze([]),
+  bodies: NO_BODIES,
+} as const)
diff --git a/mods/diff/hooks/pane-state/base-mode-of/base-mode-of.ts b/mods/diff/hooks/pane-state/base-mode-of/base-mode-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..de11c94af93ed5e26eab6a32fa7e19b4cefb0205
--- /dev/null
+++ b/mods/diff/hooks/pane-state/base-mode-of/base-mode-of.ts
@@ -0,0 +1,14 @@
+import type Git from '../../git'
+
+/**
+ * A stored or picked value as a base mode, or null when it names none.
+ *
+ * @param value what `$.store.get` returned or a Select carried
+ * @returns the mode, or null
+ */
+export function baseModeOf(value: unknown): Git.BaseMode | null {
+  const isMode =
+    value === 'session' || value === 'uncommitted' || value === 'branch'
+
+  return isMode ? value : null
+}
diff --git a/mods/diff/hooks/pane-state/base-mode-of/index.ts b/mods/diff/hooks/pane-state/base-mode-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3479978c1af1fec8fc6b4f17ddcd2eb227ebeb69
--- /dev/null
+++ b/mods/diff/hooks/pane-state/base-mode-of/index.ts
@@ -0,0 +1,3 @@
+export * from './base-mode-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/bodies/bodies.ts b/mods/diff/hooks/pane-state/bodies/bodies.ts
new file mode 100644
index 0000000000000000000000000000000000000000..91cfcf08431fe4a24bf485fbf9312b2069381388
--- /dev/null
+++ b/mods/diff/hooks/pane-state/bodies/bodies.ts
@@ -0,0 +1,7 @@
+import type Git from '../../git'
+
+/**
+ * The file bodies the pane has read, by path: the parsed hunks, or null
+ * where the backend could not read them; a path not in it is still loading.
+ */
+export type Bodies = ReadonlyMap<string, Git.FileHunks | null>
diff --git a/mods/diff/hooks/pane-state/bodies/index.ts b/mods/diff/hooks/pane-state/bodies/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..81ce6a52dd67a3f1a5f6b9fbb3f32a6ff7b2e5e5
--- /dev/null
+++ b/mods/diff/hooks/pane-state/bodies/index.ts
@@ -0,0 +1,3 @@
+export type * from './bodies.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/dialog-focus/dialog-focus.ts b/mods/diff/hooks/pane-state/dialog-focus/dialog-focus.ts
new file mode 100644
index 0000000000000000000000000000000000000000..03616566939f4037cf2e3023237abf1bdaf2d66f
--- /dev/null
+++ b/mods/diff/hooks/pane-state/dialog-focus/dialog-focus.ts
@@ -0,0 +1,8 @@
+/**
+ * What the ring landing on a dialog row settles: the file now selected,
+ * and the key of the row to land the ring on once the window re-centres.
+ */
+export type DialogFocus = {
+  selectedPath: string
+  landing: string
+}
diff --git a/mods/diff/hooks/pane-state/dialog-focus/index.ts b/mods/diff/hooks/pane-state/dialog-focus/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f85ca7bd769deae7d61e162aba8e9212a36fb6c5
--- /dev/null
+++ b/mods/diff/hooks/pane-state/dialog-focus/index.ts
@@ -0,0 +1,3 @@
+export type * from './dialog-focus.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/dialog-title/dialog-title.ts b/mods/diff/hooks/pane-state/dialog-title/dialog-title.ts
new file mode 100644
index 0000000000000000000000000000000000000000..451d87a934025794fce5ce3923dfd34813604fb6
--- /dev/null
+++ b/mods/diff/hooks/pane-state/dialog-title/dialog-title.ts
@@ -0,0 +1,8 @@
+/**
+ * The inline pane's first line, as the built-in dialog heads itself: what
+ * the rows are, then in dim what they are compared with.
+ */
+export type DialogTitle = {
+  title: string
+  subtitle: string
+}
diff --git a/mods/diff/hooks/pane-state/dialog-title/index.ts b/mods/diff/hooks/pane-state/dialog-title/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..cbe34f5c8bcaf76ec8ba51c718ab375e9b118479
--- /dev/null
+++ b/mods/diff/hooks/pane-state/dialog-title/index.ts
@@ -0,0 +1,3 @@
+export type * from './dialog-title.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/empty-state/empty-state.ts b/mods/diff/hooks/pane-state/empty-state/empty-state.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1ae9e9d2035286dd92f7bca9310a71d7c8202166
--- /dev/null
+++ b/mods/diff/hooks/pane-state/empty-state/empty-state.ts
@@ -0,0 +1,8 @@
+/**
+ * What the header says instead of a count when there is nothing to list,
+ * and the body's hint under it when the hint adds something.
+ */
+export type EmptyState = {
+  headline: string
+  hint: string | null
+}
diff --git a/mods/diff/hooks/pane-state/empty-state/index.ts b/mods/diff/hooks/pane-state/empty-state/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6a00dc2d2e04c5c04b6968c4303dbb23b4fd4fa5
--- /dev/null
+++ b/mods/diff/hooks/pane-state/empty-state/index.ts
@@ -0,0 +1,3 @@
+export type * from './empty-state.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/header-totals/header-totals.ts b/mods/diff/hooks/pane-state/header-totals/header-totals.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7bbb140936b42be0ac0374d6b2cc859d7eb10e89
--- /dev/null
+++ b/mods/diff/hooks/pane-state/header-totals/header-totals.ts
@@ -0,0 +1,10 @@
+/**
+ * The header's counts: the fetched totals less the pre-session rows, and
+ * how many session files fell past the row cap ("not shown").
+ */
+export type HeaderTotals = {
+  filesCount: number
+  linesAdded: number
+  linesRemoved: number
+  notShown: number
+}
diff --git a/mods/diff/hooks/pane-state/header-totals/index.ts b/mods/diff/hooks/pane-state/header-totals/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3b4702cac1bef13ff5038eb6b030c1084152925e
--- /dev/null
+++ b/mods/diff/hooks/pane-state/header-totals/index.ts
@@ -0,0 +1,3 @@
+export type * from './header-totals.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/initial-model/index.ts b/mods/diff/hooks/pane-state/initial-model/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..912d01fa872e3a4b56339513a9cc947b10671d6f
--- /dev/null
+++ b/mods/diff/hooks/pane-state/initial-model/index.ts
@@ -0,0 +1,3 @@
+export * from './initial-model.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/initial-model/initial-model.ts b/mods/diff/hooks/pane-state/initial-model/initial-model.ts
new file mode 100644
index 0000000000000000000000000000000000000000..069b92e84cee0ddc3b6d1d878f37597d03276b4b
--- /dev/null
+++ b/mods/diff/hooks/pane-state/initial-model/initial-model.ts
@@ -0,0 +1,29 @@
+import Git from '../../git'
+import { NO_BODIES } from '../no-bodies'
+import type { PaneModel } from '../pane-model'
+
+/**
+ * The pane before anything happened: session mode, nothing fetched,
+ * nothing picked, no body read, the current source, git's words, docked.
+ */
+export const INITIAL_MODEL: PaneModel = Object.freeze({
+  words: Git.GIT_WORDS,
+  baseModes: Git.GIT_BASE_MODES,
+  isLoading: false,
+  hasSettled: false,
+  isOutsideRepository: false,
+  data: null,
+  requestedMode: 'session',
+  selectedPath: null,
+  isNoiseShown: false,
+  isPreSessionShown: false,
+  source: Object.freeze({ kind: 'current' }),
+  turns: Object.freeze([]),
+  bodies: NO_BODIES,
+  todos: Object.freeze({ done: 0, total: 0 }),
+  armedPath: null,
+  placement: 'dock',
+  isFullscreen: null,
+  dialogView: 'list',
+  place: Object.freeze({ top: 0, listStart: 0, columns: 0, rows: 0 }),
+})
diff --git a/mods/diff/hooks/pane-state/mode-label-of/index.ts b/mods/diff/hooks/pane-state/mode-label-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..04fb5c40c413d9d7bb45b808d8cffa10281d58a4
--- /dev/null
+++ b/mods/diff/hooks/pane-state/mode-label-of/index.ts
@@ -0,0 +1,3 @@
+export * from './mode-label-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/mode-label-of/mode-label-of.ts b/mods/diff/hooks/pane-state/mode-label-of/mode-label-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..a9d22458b8c977e8df7b18a717a287fd62fe9c43
--- /dev/null
+++ b/mods/diff/hooks/pane-state/mode-label-of/mode-label-of.ts
@@ -0,0 +1,44 @@
+import type Git from '../../git'
+import type { PaneModel } from '../pane-model'
+
+/**
+ * One base mode's name in the picker, worded as the built-in's base line
+ * (diffBaseLabel) words the mode it shows.
+ *
+ * Branch mode names its base branch from the data, or once its own fetch
+ * settled without one says what stands in; uncommitted names the working
+ * tree's base. The requested mode carries an ellipsis while pending.
+ *
+ * @param mode the mode to name
+ * @param model the mode the person picked, the last good fetch, the words
+ * @returns the label
+ */
+export function modeLabelOf(
+  mode: Git.BaseMode,
+  model: Pick<PaneModel, 'requestedMode' | 'data' | 'words'>,
+): string {
+  const { data } = model
+  const source = data?.source
+  const isPending = data !== null && model.requestedMode !== data.mode
+  const isRequested = mode === model.requestedMode
+  const suffix = isRequested && isPending ? '…' : ''
+  const base = source?.kind === 'working-tree' ? source.base : model.words.base
+  const isSettledBranch = isRequested && !isPending && data?.mode === 'branch'
+
+  function nameOf(): string {
+    switch (mode) {
+      case 'session':
+        return 'this session'
+      case 'uncommitted':
+        return `uncommitted (vs ${base})`
+      case 'branch':
+        if (source?.kind === 'branch') {
+          return `branch vs ${source.baseBranch}`
+        }
+
+        return isSettledBranch ? `vs ${base} (no base branch)` : 'branch diff'
+    }
+  }
+
+  return `${nameOf()}${suffix}`
+}
diff --git a/mods/diff/hooks/pane-state/no-bodies/index.ts b/mods/diff/hooks/pane-state/no-bodies/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..43ea111f7394ebd9bbe7616ce6a6172414e3c6e2
--- /dev/null
+++ b/mods/diff/hooks/pane-state/no-bodies/index.ts
@@ -0,0 +1,3 @@
+export * from './no-bodies.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/no-bodies/no-bodies.ts b/mods/diff/hooks/pane-state/no-bodies/no-bodies.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9bf8be4c9ef290caf5065e6786a9d2434d25e9d4
--- /dev/null
+++ b/mods/diff/hooks/pane-state/no-bodies/no-bodies.ts
@@ -0,0 +1,7 @@
+import type { Bodies } from '../bodies'
+
+/**
+ * No body read yet: the pane's bodies before a fetch, and after one whose
+ * base moved.
+ */
+export const NO_BODIES: Bodies = new Map()
diff --git a/mods/diff/hooks/pane-state/pane-model/index.ts b/mods/diff/hooks/pane-state/pane-model/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4195968877b79a6b9c51ebb5f1259234e8ac92d1
--- /dev/null
+++ b/mods/diff/hooks/pane-state/pane-model/index.ts
@@ -0,0 +1,3 @@
+export type * from './pane-model.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/pane-model/pane-model.ts b/mods/diff/hooks/pane-state/pane-model/pane-model.ts
new file mode 100644
index 0000000000000000000000000000000000000000..905a6f6af0ea8387095c7aa4885c3cc0ccc15fba
--- /dev/null
+++ b/mods/diff/hooks/pane-state/pane-model/pane-model.ts
@@ -0,0 +1,36 @@
+import type Backend from '../../backend'
+import type Git from '../../git'
+import type Todos from '../../todos'
+import type Turns from '../../turns'
+import type { Bodies } from '../bodies'
+import type { Source } from '../source'
+
+/**
+ * Everything one drawing of the pane reads.
+ *
+ * The last good fetch, what the person picked, the bodies read so far, the
+ * turns and todos, how the surface seated it, whether the layout docks a
+ * pane (once a drawing or `/diff` said), the dialog's view, and the docked
+ * scroll in its last box.
+ */
+export type PaneModel = {
+  words: Backend.BackendWords
+  baseModes: readonly Git.BaseMode[]
+  isLoading: boolean
+  hasSettled: boolean
+  isOutsideRepository: boolean
+  data: Git.DiffData | null
+  requestedMode: Git.BaseMode
+  selectedPath: string | null
+  isNoiseShown: boolean
+  isPreSessionShown: boolean
+  source: Source
+  turns: readonly Turns.TurnDiff[]
+  bodies: Bodies
+  todos: Todos.TodoProgress
+  armedPath: string | null
+  placement: 'dock' | 'inline'
+  isFullscreen: boolean | null
+  dialogView: 'list' | 'detail'
+  place: { top: number; listStart: number; columns: number; rows: number }
+}
diff --git a/mods/diff/hooks/pane-state/partition/index.ts b/mods/diff/hooks/pane-state/partition/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..993d49fef0c557abdb007d002a9c8a13cf054de4
--- /dev/null
+++ b/mods/diff/hooks/pane-state/partition/index.ts
@@ -0,0 +1,3 @@
+export type * from './partition.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/partition/partition.ts b/mods/diff/hooks/pane-state/partition/partition.ts
new file mode 100644
index 0000000000000000000000000000000000000000..befaaee5584e1b9d3ede981cabd17187b9bce937
--- /dev/null
+++ b/mods/diff/hooks/pane-state/partition/partition.ts
@@ -0,0 +1,11 @@
+import type Git from '../../git'
+
+/**
+ * A fetch's rows as the pane groups them: the session's own that are
+ * listed, how many tests and generated files there are, the pre-session.
+ */
+export type Partition = {
+  shown: readonly Git.FileStat[]
+  preSession: readonly Git.FileStat[]
+  noiseCount: number
+}
diff --git a/mods/diff/hooks/pane-state/picked-turn-of/index.ts b/mods/diff/hooks/pane-state/picked-turn-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..024515313df7c9dc151c133406f9590fb9407915
--- /dev/null
+++ b/mods/diff/hooks/pane-state/picked-turn-of/index.ts
@@ -0,0 +1,3 @@
+export * from './picked-turn-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/picked-turn-of/picked-turn-of.ts b/mods/diff/hooks/pane-state/picked-turn-of/picked-turn-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b83f52715b541ad44422aab7737fcef913963000
--- /dev/null
+++ b/mods/diff/hooks/pane-state/picked-turn-of/picked-turn-of.ts
@@ -0,0 +1,16 @@
+import type Turns from '../../turns'
+import type { PaneModel } from '../pane-model'
+
+/**
+ * The turn the pane's source names while the transcript still holds it;
+ * undefined for the current source or a turn a rewind dropped.
+ *
+ * @param model the pane's state
+ * @returns the picked turn, or undefined
+ */
+export function pickedTurnOf(model: PaneModel): Turns.TurnDiff | undefined {
+  const { source } = model
+  const wanted = source.kind === 'turn' ? source.index : null
+
+  return model.turns.find(candidate => candidate.index === wanted)
+}
diff --git a/mods/diff/hooks/pane-state/source/index.ts b/mods/diff/hooks/pane-state/source/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f1b0da3a89156d43838ffdd9e1bba6cc07027e56
--- /dev/null
+++ b/mods/diff/hooks/pane-state/source/index.ts
@@ -0,0 +1,3 @@
+export type * from './source.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/source/source.ts b/mods/diff/hooks/pane-state/source/source.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e5621a26b3d4bb0485148187df7444329b50ece1
--- /dev/null
+++ b/mods/diff/hooks/pane-state/source/source.ts
@@ -0,0 +1,4 @@
+/**
+ * What the pane lists: the repository's diff now, or one past turn's edits.
+ */
+export type Source = { kind: 'current' } | { kind: 'turn'; index: number }
diff --git a/mods/diff/hooks/pane-state/turn-title-of/index.ts b/mods/diff/hooks/pane-state/turn-title-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0d068ba1bd1e309c014fad20ce77519ad3e21a03
--- /dev/null
+++ b/mods/diff/hooks/pane-state/turn-title-of/index.ts
@@ -0,0 +1,3 @@
+export * from './turn-title-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/turn-title-of/turn-title-of.ts b/mods/diff/hooks/pane-state/turn-title-of/turn-title-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..92db84d1887c31a94c54a750d713399f5d72c783
--- /dev/null
+++ b/mods/diff/hooks/pane-state/turn-title-of/turn-title-of.ts
@@ -0,0 +1,16 @@
+import type Turns from '../../turns'
+import type { DialogTitle } from '../dialog-title'
+
+/**
+ * A picked turn's heading, as the built-in dialog titles its tab: the
+ * turn's number, then its prompt's opening words in quotes when it has any.
+ *
+ * @param turn the turn
+ * @returns the title and its dim subtitle
+ */
+export const turnTitleOf = (
+  turn: Pick<Turns.TurnDiff, 'index' | 'preview'>,
+): DialogTitle => ({
+  title: `Turn ${turn.index}`,
+  subtitle: turn.preview === '' ? '' : `"${turn.preview}"`,
+})
diff --git a/mods/diff/hooks/pane-state/turn-totals-of/index.ts b/mods/diff/hooks/pane-state/turn-totals-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e33459e007ae721d9a9d6ecf0237a5cd7c067485
--- /dev/null
+++ b/mods/diff/hooks/pane-state/turn-totals-of/index.ts
@@ -0,0 +1,3 @@
+export * from './turn-totals-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/turn-totals-of/turn-totals-of.ts b/mods/diff/hooks/pane-state/turn-totals-of/turn-totals-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ea49e80a3bea36563deba4655202ec52bbc3aa73
--- /dev/null
+++ b/mods/diff/hooks/pane-state/turn-totals-of/turn-totals-of.ts
@@ -0,0 +1,16 @@
+import type Turns from '../../turns'
+import type { HeaderTotals } from '../header-totals'
+
+/**
+ * A turn's header counts: its files and their summed lines; nothing falls
+ * past a cap.
+ *
+ * @param turn the turn
+ * @returns the counts
+ */
+export const turnTotalsOf = (turn: Turns.TurnDiff): HeaderTotals => ({
+  filesCount: turn.files.length,
+  linesAdded: turn.files.reduce((sum, file) => sum + file.added, 0),
+  linesRemoved: turn.files.reduce((sum, file) => sum + file.removed, 0),
+  notShown: 0,
+})
diff --git a/mods/diff/hooks/pane-state/zero-totals/index.ts b/mods/diff/hooks/pane-state/zero-totals/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..59d381c3690a6b8eb1e1baf3da25de43af99806f
--- /dev/null
+++ b/mods/diff/hooks/pane-state/zero-totals/index.ts
@@ -0,0 +1,3 @@
+export * from './zero-totals.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-state/zero-totals/zero-totals.ts b/mods/diff/hooks/pane-state/zero-totals/zero-totals.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9c372e1a11238c08e6481b6e52f19289731ee361
--- /dev/null
+++ b/mods/diff/hooks/pane-state/zero-totals/zero-totals.ts
@@ -0,0 +1,12 @@
+import type { HeaderTotals } from '../header-totals'
+
+/**
+ * The header's counts when no fetch has settled into data: nothing
+ * changed, nothing past the cap.
+ */
+export const ZERO_TOTALS: HeaderTotals = Object.freeze({
+  filesCount: 0,
+  linesAdded: 0,
+  linesRemoved: 0,
+  notShown: 0,
+})
diff --git a/mods/diff/hooks/pane-toggle/pane-belief/index.ts b/mods/diff/hooks/pane-toggle/pane-belief/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..921fcf308ab415bc9da9d03437b412464c0b775a
--- /dev/null
+++ b/mods/diff/hooks/pane-toggle/pane-belief/index.ts
@@ -0,0 +1,3 @@
+export type * from './pane-belief.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/pane-toggle/pane-belief/pane-belief.ts b/mods/diff/hooks/pane-toggle/pane-belief/pane-belief.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e965cc1f946920df28188ea7d2053e0fae043222
--- /dev/null
+++ b/mods/diff/hooks/pane-toggle/pane-belief/pane-belief.ts
@@ -0,0 +1,11 @@
+/**
+ * What the plugin knows of its pane when `/diff` runs.
+ *
+ * Whether the pane is open (it opened it, and no close of any origin has
+ * gone through since), and the terminal's width as last drawn (null before
+ * any draw).
+ */
+export type PaneBelief = {
+  isOpen: boolean
+  columns: number | null
+}
diff --git a/mods/diff/hooks/record/base-outcomes/base-outcomes.ts b/mods/diff/hooks/record/base-outcomes/base-outcomes.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0b4a2bf4f39d1166d2cdc174db9ccc3c6fb14be3
--- /dev/null
+++ b/mods/diff/hooks/record/base-outcomes/base-outcomes.ts
@@ -0,0 +1,5 @@
+/**
+ * What branch mode resolved its base to, as a base-resolve mark names it,
+ * the built-in panel's own: the merge base, HEAD itself, or nothing.
+ */
+export const BASE_OUTCOMES = ['merge-base', 'head-is-base', 'none'] as const
diff --git a/mods/diff/hooks/record/base-outcomes/index.ts b/mods/diff/hooks/record/base-outcomes/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..54bae72a4b9f9c0e07d9318791a35004a5ab2b40
--- /dev/null
+++ b/mods/diff/hooks/record/base-outcomes/index.ts
@@ -0,0 +1,3 @@
+export * from './base-outcomes.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/features/features.ts b/mods/diff/hooks/record/features/features.ts
new file mode 100644
index 0000000000000000000000000000000000000000..bc245623bc675bf9995eb30b61070fe6725bb467
--- /dev/null
+++ b/mods/diff/hooks/record/features/features.ts
@@ -0,0 +1,16 @@
+/**
+ * The feature names the plugin marks, kept as the built-in panel's so the
+ * feature surface keeps one series across the swap.
+ *
+ * `read` per fetch round, `tabSwitch` per `/diff` open or close and per
+ * close by the pane's own control, `baseSwitch` per base pick, `baseResolve`
+ * on the first resolve and the first failure, `selectionAttach` per diff
+ * asked.
+ */
+export const FEATURES = {
+  read: 'repl_diff_read',
+  tabSwitch: 'repl_tab_switch',
+  baseSwitch: 'repl_diff_base_switch',
+  baseResolve: 'diff_base_resolve',
+  selectionAttach: 'diff_selection_attach',
+} as const
diff --git a/mods/diff/hooks/record/features/index.ts b/mods/diff/hooks/record/features/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..672d470c52134d6c2991012bb72f6fbbc3f6c48f
--- /dev/null
+++ b/mods/diff/hooks/record/features/index.ts
@@ -0,0 +1,3 @@
+export * from './features.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/mark-outcome/index.ts b/mods/diff/hooks/record/mark-outcome/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d6e1809167bacae05f907474cb7b61dfb449d337
--- /dev/null
+++ b/mods/diff/hooks/record/mark-outcome/index.ts
@@ -0,0 +1,3 @@
+export type * from './mark-outcome.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/mark-outcome/mark-outcome.ts b/mods/diff/hooks/record/mark-outcome/mark-outcome.ts
new file mode 100644
index 0000000000000000000000000000000000000000..06225a06d286b71ec4aaacbb2557c7185ee87431
--- /dev/null
+++ b/mods/diff/hooks/record/mark-outcome/mark-outcome.ts
@@ -0,0 +1,12 @@
+import type { Host } from '../../host'
+
+/**
+ * How a marked feature went: fine, or degraded for a reason, with the
+ * properties the built-in panel's mark carries beside it.
+ *
+ * The pane never marks `bad`: a failed fetch keeps the last good diff on
+ * screen.
+ */
+export type MarkOutcome = ({ kind: 'ok' } | { kind: 'sad'; reason: string }) & {
+  props?: Parameters<Host['mark']>[0]['props']
+}
diff --git a/mods/diff/hooks/record/recorder/index.ts b/mods/diff/hooks/record/recorder/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ffa2cf745b739dd89d5f1b58ccc1fc4b24acb2e4
--- /dev/null
+++ b/mods/diff/hooks/record/recorder/index.ts
@@ -0,0 +1,3 @@
+export type * from './recorder.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/recorder/recorder.ts b/mods/diff/hooks/record/recorder/recorder.ts
new file mode 100644
index 0000000000000000000000000000000000000000..252e70474d5757155d8e138119bc6a9b7a711e3d
--- /dev/null
+++ b/mods/diff/hooks/record/recorder/recorder.ts
@@ -0,0 +1,28 @@
+import type { MarkOutcome } from '../mark-outcome'
+import type { SHOWN_TRIGGERS } from '../shown-triggers'
+import type { WIDTH_BUCKETS } from '../width-buckets'
+
+/**
+ * The plugin's telemetry as its hooks call it; every method swallows a
+ * missing `$.telemetry` and a refused row, so recording never disturbs.
+ */
+export type Recorder = {
+  /**
+   * One `tengu_feature_<kind>` row.
+   */
+  mark: (feature: string, outcome: MarkOutcome) => void
+
+  /**
+   * The built-in panel's `tengu_repl_diff_panel_shown` row, once a session.
+   */
+  shown: (
+    trigger: (typeof SHOWN_TRIGGERS)[number],
+    bucket: (typeof WIDTH_BUCKETS)[number],
+  ) => void
+
+  /**
+   * The built-in's `diff_selection_attach` feature mark: a file's diff rode
+   * the next prompt.
+   */
+  asked: () => void
+}
diff --git a/mods/diff/hooks/record/safely/index.ts b/mods/diff/hooks/record/safely/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..002c90f9e8a5d2ae9607a656527aca5d11f6f958
--- /dev/null
+++ b/mods/diff/hooks/record/safely/index.ts
@@ -0,0 +1,3 @@
+export * from './safely.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/safely/safely.ts b/mods/diff/hooks/record/safely/safely.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8bda37a758e9a7a70dd205e477d46f5ee2141ff0
--- /dev/null
+++ b/mods/diff/hooks/record/safely/safely.ts
@@ -0,0 +1,11 @@
+/**
+ * Sends one telemetry row and forgets it: a missing `$.telemetry` (no
+ * telemetry built-in) throws synchronously, a refused row rejects; both drop.
+ *
+ * @param send posts the row
+ */
+export function safely(send: () => Promise<void>) {
+  try {
+    void send().catch(() => undefined)
+  } catch {}
+}
diff --git a/mods/diff/hooks/record/shown-triggers/index.ts b/mods/diff/hooks/record/shown-triggers/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0c963ba667b0ac62787801cb9b67260c20109838
--- /dev/null
+++ b/mods/diff/hooks/record/shown-triggers/index.ts
@@ -0,0 +1,3 @@
+export * from './shown-triggers.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/shown-triggers/shown-triggers.ts b/mods/diff/hooks/record/shown-triggers/shown-triggers.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fdb1a80e1e9805de00c48e4c74711ae6e080ce18
--- /dev/null
+++ b/mods/diff/hooks/record/shown-triggers/shown-triggers.ts
@@ -0,0 +1,5 @@
+/**
+ * What opened the pane the first time it showed this session: Claude's
+ * first edit, or the person's `/diff`.
+ */
+export const SHOWN_TRIGGERS = ['auto_open', 'manual'] as const
diff --git a/mods/diff/hooks/record/tabs/index.ts b/mods/diff/hooks/record/tabs/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..27f7c5bb8aeca418fe24c234bd9d3219de325353
--- /dev/null
+++ b/mods/diff/hooks/record/tabs/index.ts
@@ -0,0 +1,3 @@
+export * from './tabs.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/tabs/tabs.ts b/mods/diff/hooks/record/tabs/tabs.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6a614b47692bdd5b4e6ae83ed6693a0392432153
--- /dev/null
+++ b/mods/diff/hooks/record/tabs/tabs.ts
@@ -0,0 +1,5 @@
+/**
+ * Where a tab-switch mark says the person went, named as the built-in
+ * panel names it: the diff, or the conversation it closes back to.
+ */
+export const TABS = ['diff', 'convo'] as const
diff --git a/mods/diff/hooks/record/width-bucket-of/index.ts b/mods/diff/hooks/record/width-bucket-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9127054c1c3ef7887cbbc2bf52a537acdb79ed03
--- /dev/null
+++ b/mods/diff/hooks/record/width-bucket-of/index.ts
@@ -0,0 +1,3 @@
+export * from './width-bucket-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/width-bucket-of/width-bucket-of.ts b/mods/diff/hooks/record/width-bucket-of/width-bucket-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d98208e38e132446ab4a9c99a6b35ae5bb971521
--- /dev/null
+++ b/mods/diff/hooks/record/width-bucket-of/width-bucket-of.ts
@@ -0,0 +1,21 @@
+import Limits from '../../limits'
+import { WIDTH_BUCKETS } from '../width-buckets'
+
+/**
+ * The bucket a terminal width falls in, the built-in panel's own buckets.
+ *
+ * @param columns the terminal's width, or null when no render told it yet
+ * @returns one of WIDTH_BUCKETS
+ */
+export function widthBucketOf(
+  columns: number | null,
+): (typeof WIDTH_BUCKETS)[number] {
+  const [narrow, medium, wide, widest] = WIDTH_BUCKETS
+  const width = columns ?? 0
+
+  const isNarrow = width < Limits.OPEN_MIN_COLUMNS
+  const isMedium = width < Limits.AUTO_OPEN_MIN_COLUMNS
+  const isWide = width < Limits.WIDEST_BUCKET_COLUMNS
+
+  return isNarrow ? narrow : isMedium ? medium : isWide ? wide : widest
+}
diff --git a/mods/diff/hooks/record/width-buckets/index.ts b/mods/diff/hooks/record/width-buckets/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..14a139cd9c954b25b7d11db94e72b9219cca6f39
--- /dev/null
+++ b/mods/diff/hooks/record/width-buckets/index.ts
@@ -0,0 +1,3 @@
+export * from './width-buckets.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/record/width-buckets/width-buckets.ts b/mods/diff/hooks/record/width-buckets/width-buckets.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b3132d63345a0531c4b806ad79a5f18f667bce5f
--- /dev/null
+++ b/mods/diff/hooks/record/width-buckets/width-buckets.ts
@@ -0,0 +1,10 @@
+/**
+ * The terminal-width buckets the pane-shown row reports, the built-in
+ * panel's own.
+ */
+export const WIDTH_BUCKETS = [
+  'under_110',
+  '110_to_143',
+  '144_to_199',
+  '200_plus',
+] as const
diff --git a/mods/diff/hooks/todos/todo-progress/index.ts b/mods/diff/hooks/todos/todo-progress/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0ab3b873448b83edd36d62bd65eb7bbc178d38e6
--- /dev/null
+++ b/mods/diff/hooks/todos/todo-progress/index.ts
@@ -0,0 +1,3 @@
+export type * from './todo-progress.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/todos/todo-progress/todo-progress.ts b/mods/diff/hooks/todos/todo-progress/todo-progress.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b053fa2cc52baca3b88e90d56a2c3b8aa9217df1
--- /dev/null
+++ b/mods/diff/hooks/todos/todo-progress/todo-progress.ts
@@ -0,0 +1,7 @@
+/**
+ * How far the session's todo list has come: completed items over all.
+ */
+export type TodoProgress = {
+  done: number
+  total: number
+}
diff --git a/mods/diff/hooks/turns/turn-of/file-edit-of/file-edit-of.ts b/mods/diff/hooks/turns/turn-of/file-edit-of/file-edit-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..2a68d229ff38358f31cb6bbac6e838147141f39e
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/file-edit-of/file-edit-of.ts
@@ -0,0 +1,53 @@
+import { countOf } from '../../../count-of'
+import { isRecord } from '../../../is-record'
+import type Types from '../../types'
+import { hunksOf } from './hunks-of'
+
+/**
+ * One Edit or Write result as the file it changed, told as the built-in
+ * turn view tells an edit result; null for any other result.
+ *
+ * A `filePath` with a non-empty `structuredPatch`, or a `create` with its
+ * `content`, which becomes one all-`+` hunk.
+ *
+ * @param result what the transcript stored for the tool call
+ * @returns the file's edit, or null
+ */
+export function fileEditOf(result: unknown): Types.TurnFile | null {
+  if (!isRecord(result) || typeof result.filePath !== 'string') {
+    return null
+  }
+
+  const hunks = hunksOf(result.structuredPatch)
+  const isNewFile = result.type === 'create'
+
+  if (hunks.length > 0) {
+    const lines = hunks.flatMap(hunk => hunk.lines)
+
+    return {
+      path: result.filePath,
+      hunks,
+      added: countOf(lines, line => line.startsWith('+')),
+      removed: countOf(lines, line => line.startsWith('-')),
+      isNewFile,
+      isTruncated: false,
+    }
+  }
+
+  if (!isNewFile || typeof result.content !== 'string') {
+    return null
+  }
+
+  const created = result.content.split('\n')
+
+  return {
+    path: result.filePath,
+    hunks: [
+      { oldStart: 0, newStart: 1, lines: created.map(line => `+${line}`) },
+    ],
+    added: created.length,
+    removed: 0,
+    isNewFile: true,
+    isTruncated: false,
+  }
+}
diff --git a/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/hunks-of.ts b/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/hunks-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ec1967e6fb22d0dfe356f03dc56226b931a0fd33
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/hunks-of.ts
@@ -0,0 +1,34 @@
+import { arrayOf } from '../../../../array-of'
+import type Git from '../../../../git'
+import { isRecord } from '../../../../is-record'
+
+/**
+ * A tool result's `structuredPatch` as hunks, keeping only entries shaped
+ * like one (numeric starts, string lines).
+ *
+ * @param patch the result's `structuredPatch` field
+ * @returns the well-formed hunks; empty when the field is not an array
+ */
+export const hunksOf = (patch: unknown): Git.Hunk[] =>
+  arrayOf(patch)
+    .filter(isRecord)
+    .flatMap(hunk => {
+      const { oldStart, newStart, lines } = hunk
+
+      const isHunk =
+        typeof oldStart === 'number' &&
+        typeof newStart === 'number' &&
+        Array.isArray(lines)
+
+      return isHunk
+        ? [
+            {
+              oldStart,
+              newStart,
+              lines: arrayOf(lines).filter(
+                (line): line is string => typeof line === 'string',
+              ),
+            },
+          ]
+        : []
+    })
diff --git a/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/index.ts b/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..a106cc06dacebcdda2e863e6a1bab99bbe335ce6
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/file-edit-of/hunks-of/index.ts
@@ -0,0 +1,3 @@
+export * from './hunks-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/turn-of/file-edit-of/index.ts b/mods/diff/hooks/turns/turn-of/file-edit-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b03ef90492eb571b6ad20b1dfc2fda68c580c0c4
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/file-edit-of/index.ts
@@ -0,0 +1,4 @@
+export * from './file-edit-of.js'
+export * from './hunks-of'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/turn-of/index.ts b/mods/diff/hooks/turns/turn-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..33f8048a19db3a8efc411e362b4d2c35ba9022c2
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/index.ts
@@ -0,0 +1,6 @@
+export * from './file-edit-of'
+export * from './is-prompt-row'
+export * from './preview-chars'
+export * from './turn-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/turn-of/is-prompt-row/index.ts b/mods/diff/hooks/turns/turn-of/is-prompt-row/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..486694e91d0025f7eb03249e6ee044e6b40d7042
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/is-prompt-row/index.ts
@@ -0,0 +1,3 @@
+export * from './is-prompt-row.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/turn-of/is-prompt-row/is-prompt-row.ts b/mods/diff/hooks/turns/turn-of/is-prompt-row/is-prompt-row.ts
new file mode 100644
index 0000000000000000000000000000000000000000..14155f7e06f9280b9cb7a34ef2ed62ae2d204f4c
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/is-prompt-row/is-prompt-row.ts
@@ -0,0 +1,13 @@
+import type { SessionMessage } from 'claude-code'
+
+/**
+ * Whether a transcript row starts a turn: a user message that carries text
+ * and no tool results (`$.session.messages()` already leaves out meta rows).
+ *
+ * @param message one row of the transcript
+ * @returns whether it is a typed prompt
+ */
+export const isPromptRow = (message: SessionMessage) =>
+  message.role === 'user' &&
+  (message.toolResults === undefined || message.toolResults.length === 0) &&
+  message.text !== ''
diff --git a/mods/diff/hooks/turns/turn-of/preview-chars/index.ts b/mods/diff/hooks/turns/turn-of/preview-chars/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..114ed80bbc804877e46abaf14f235e6896e539ca
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/preview-chars/index.ts
@@ -0,0 +1,3 @@
+export * from './preview-chars.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/turn-of/preview-chars/preview-chars.ts b/mods/diff/hooks/turns/turn-of/preview-chars/preview-chars.ts
new file mode 100644
index 0000000000000000000000000000000000000000..883d6038cb3237b9ca16f6187ec077c07958721f
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/preview-chars/preview-chars.ts
@@ -0,0 +1,5 @@
+/**
+ * How many characters of a turn's prompt its source label keeps, as the
+ * built-in turn view does: 30, an ellipsis standing for the rest.
+ */
+export const PREVIEW_CHARS = 30
diff --git a/mods/diff/hooks/turns/turn-of/turn-of.ts b/mods/diff/hooks/turns/turn-of/turn-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..dac2c73afc15f37af51d7bd4005e34705dc3d4fd
--- /dev/null
+++ b/mods/diff/hooks/turns/turn-of/turn-of.ts
@@ -0,0 +1,61 @@
+import type { SessionMessage } from 'claude-code'
+
+import Git from '../../git'
+import type Types from '../types'
+import { fileEditOf } from './file-edit-of'
+import { PREVIEW_CHARS } from './preview-chars'
+
+/**
+ * One turn's rows (its prompt first) as the files its tool calls edited,
+ * as the built-in turn view reads them.
+ *
+ * A file edited twice keeps both calls' hunks (cut at MAX_LINES_PER_FILE
+ * in all) and the summed counts, and one created then edited stays new.
+ *
+ * @param rows the prompt row through the last row before the next prompt
+ * @param index the turn's number, from 1
+ * @returns the turn; `files` empty when nothing was edited
+ */
+export function turnOf(
+  rows: readonly SessionMessage[],
+  index: number,
+): Types.TurnDiff {
+  const text = rows[0]?.text ?? ''
+  const isShort = [...text].length <= PREVIEW_CHARS
+
+  const preview = isShort
+    ? text
+    : `${[...text].slice(0, PREVIEW_CHARS - 1).join('')}…`
+
+  const edits = rows
+    .flatMap(row => (row.role === 'assistant' ? row.toolUses : []))
+    .map(use => fileEditOf(use.result))
+    .filter((edit): edit is Types.TurnFile => edit !== null)
+
+  const byPath = new Map<string, Types.TurnFile>()
+
+  for (const edit of edits) {
+    const earlier = byPath.get(edit.path)
+
+    byPath.set(
+      edit.path,
+      earlier
+        ? {
+            path: edit.path,
+            hunks: [...earlier.hunks, ...edit.hunks],
+            added: earlier.added + edit.added,
+            removed: earlier.removed + edit.removed,
+            isNewFile: earlier.isNewFile || edit.isNewFile,
+            isTruncated: false,
+          }
+        : edit,
+    )
+  }
+
+  const files = [...byPath.values()].map(file => ({
+    ...file,
+    ...Git.cutHunks(file.hunks),
+  }))
+
+  return { index, preview, files }
+}
diff --git a/mods/diff/hooks/turns/types/index.ts b/mods/diff/hooks/turns/types/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3c5ba82bf74441b09abd638e3a379a530434c147
--- /dev/null
+++ b/mods/diff/hooks/turns/types/index.ts
@@ -0,0 +1,4 @@
+export type * from './turn-diff.js'
+export * from './turn-file'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/types/turn-diff.ts b/mods/diff/hooks/turns/types/turn-diff.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ac0bd7b60424e6ce91838e17e4da3a558b763d90
--- /dev/null
+++ b/mods/diff/hooks/turns/types/turn-diff.ts
@@ -0,0 +1,11 @@
+import type { TurnFile } from './turn-file'
+
+/**
+ * The file edits of one turn: a prompt the person typed through the tool
+ * results that followed it, numbered from 1 in transcript order.
+ */
+export type TurnDiff = {
+  index: number
+  preview: string
+  files: readonly TurnFile[]
+}
diff --git a/mods/diff/hooks/turns/types/turn-file/index.ts b/mods/diff/hooks/turns/types/turn-file/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..abb92a493729c703042eaf303731ee85c9b54e9a
--- /dev/null
+++ b/mods/diff/hooks/turns/types/turn-file/index.ts
@@ -0,0 +1,3 @@
+export type * from './turn-file.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/turns/types/turn-file/turn-file.ts b/mods/diff/hooks/turns/types/turn-file/turn-file.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3d1154b389a3e72435be02b00145f5a79c633303
--- /dev/null
+++ b/mods/diff/hooks/turns/types/turn-file/turn-file.ts
@@ -0,0 +1,18 @@
+import type Git from '../../../git'
+
+/**
+ * One file as a turn's edits left it: its hunks (cut at the pane's line
+ * cap), the counts over every line, and whether the turn created it.
+ */
+export type TurnFile = {
+  path: string
+  hunks: readonly Git.Hunk[]
+  added: number
+  removed: number
+  isNewFile: boolean
+
+  /**
+   * Whether the hunks were cut at MAX_LINES_PER_FILE lines.
+   */
+  isTruncated: boolean
+}
diff --git a/mods/diff/hooks/views/dialog-pane/dialog-entries-of/dialog-entries-of.ts b/mods/diff/hooks/views/dialog-pane/dialog-entries-of/dialog-entries-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f5eb25867d79e1026fc5530827bf32cd86bfc86d
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/dialog-entries-of/dialog-entries-of.ts
@@ -0,0 +1,24 @@
+import Git from '../../../git'
+import PaneState from '../../../pane-state'
+import Entries from '../../entries'
+
+/**
+ * The files the dialog lists, in its order: the picked turn's, or every
+ * file of the fetch, pre-session or not, sorted by its displayed name.
+ *
+ * @param model the pane's state
+ * @returns the entries
+ */
+export function dialogEntriesOf(
+  model: PaneState.PaneModel,
+): readonly Entries.BodyEntry[] {
+  const turn = PaneState.pickedTurnOf(model)
+
+  return turn
+    ? turn.files.map(Entries.turnEntryOf)
+    : [...(model.data?.files ?? [])]
+        .sort((a, b) =>
+          Git.displayPathOf(a).localeCompare(Git.displayPathOf(b)),
+        )
+        .map(file => Entries.bodyEntryOf(file, model))
+}
diff --git a/mods/diff/hooks/views/dialog-pane/dialog-entries-of/index.ts b/mods/diff/hooks/views/dialog-pane/dialog-entries-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7151916fe162f00ee44a0e240127d0bb9c5af453
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/dialog-entries-of/index.ts
@@ -0,0 +1,3 @@
+export * from './dialog-entries-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/dialog-window-of/dialog-window-of.ts b/mods/diff/hooks/views/dialog-pane/dialog-window-of/dialog-window-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..71090dc99109780f7f815220bbc6c1a5fb6f92a4
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/dialog-window-of/dialog-window-of.ts
@@ -0,0 +1,23 @@
+import Limits from '../../../limits'
+import type PaneState from '../../../pane-state'
+
+/**
+ * The first of the MAX_VISIBLE_FILES rows the dialog lists (DiffFileList's
+ * window): centred on the selected file, the first when none is, clamped.
+ *
+ * @param model the pane's state: the selection
+ * @param paths the listed files' paths, in order
+ * @returns the first listed row's index
+ */
+export const dialogWindowOf = (
+  model: Pick<PaneState.PaneModel, 'selectedPath'>,
+  paths: readonly string[],
+): number =>
+  Math.max(
+    0,
+    Math.min(
+      paths.length - Limits.MAX_VISIBLE_FILES,
+      Math.max(0, paths.indexOf(model.selectedPath ?? '')) -
+        Math.floor(Limits.MAX_VISIBLE_FILES / 2),
+    ),
+  )
diff --git a/mods/diff/hooks/views/dialog-pane/dialog-window-of/index.ts b/mods/diff/hooks/views/dialog-pane/dialog-window-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f50f3d1c39c22074e0ca8a3766d4c2baeaf670b6
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/dialog-window-of/index.ts
@@ -0,0 +1,3 @@
+export * from './dialog-window-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/list-body-of/index.ts b/mods/diff/hooks/views/dialog-pane/list-body-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..96fa17f5336a67aa92873400d512f88a2e0a7e0e
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/list-body-of/index.ts
@@ -0,0 +1,4 @@
+export * from './list-body-of.js'
+export * from './list-place'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/list-body-of/list-body-of.ts b/mods/diff/hooks/views/dialog-pane/list-body-of/list-body-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..5273ab03ea1ff8d6adadadd5521c641301250a5f
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/list-body-of/list-body-of.ts
@@ -0,0 +1,43 @@
+import type Git from '../../../git'
+import type { ListPlace } from './list-place'
+
+/**
+ * The list's body as ReplDiffSidebarBody decides it: the empty state's
+ * hint, the too-many state, the only-noise state, or the session's rows.
+ *
+ * Strings are dim lines; rows are files to draw.
+ *
+ * @param place the fetch, its groups, the counts, the empty state
+ * @returns the lines and rows, in order
+ */
+export function listBodyOf(
+  place: ListPlace,
+): readonly (string | Git.FileStat)[] {
+  const { data, partition, totals, empty } = place
+
+  const hint = empty?.hint ?? null
+
+  if (empty) {
+    return hint === null ? [] : [hint]
+  }
+
+  const isTooMany =
+    data !== null && data.files.length === 0 && totals.filesCount > 0
+
+  if (isTooMany) {
+    return [
+      'Too many changed files to show diff',
+      'Per-file diff is skipped above 500 files',
+    ]
+  }
+
+  const isOnlyNoise = partition.shown.length === 0 && partition.noiseCount > 0
+
+  return isOnlyNoise
+    ? [
+        'Only tests and generated files changed',
+        'Tests and generated files are hidden · click "show" above to view ' +
+          'them',
+      ]
+    : partition.shown
+}
diff --git a/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/index.ts b/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fc5d4cf1070e442211d5774070ebcd71120f6101
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/index.ts
@@ -0,0 +1,3 @@
+export type * from './list-place.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/list-place.ts b/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/list-place.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e517bde232095c3da57c6c421f55ff5f323e9b3d
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/list-body-of/list-place/list-place.ts
@@ -0,0 +1,13 @@
+import type Git from '../../../../git'
+import type PaneState from '../../../../pane-state'
+
+/**
+ * What decides the list's body: the fetch, its grouped rows, the header's
+ * counts, and the empty state when there is one.
+ */
+export type ListPlace = {
+  data: Git.DiffData | null
+  partition: PaneState.Partition
+  totals: PaneState.HeaderTotals
+  empty: PaneState.EmptyState | null
+}
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/foot-rows.ts b/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/foot-rows.ts
new file mode 100644
index 0000000000000000000000000000000000000000..60dcdcc5b4524ce232a37dcafd510a9a11301639
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/foot-rows.ts
@@ -0,0 +1,5 @@
+/**
+ * The rows the pre-session line takes at the pane's foot: a blank row, the
+ * line, and a blank row under it.
+ */
+export const FOOT_ROWS = 3
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/index.ts b/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..2718b9b52c13be4eab5b5b3decc988815c352bb7
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/foot-rows/index.ts
@@ -0,0 +1,3 @@
+export * from './foot-rows.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/index.ts b/mods/diff/hooks/views/dialog-pane/message-pane-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8a4704e07aa001d15671b4074182653d313a0f4b
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/index.ts
@@ -0,0 +1,5 @@
+export * from './foot-rows'
+export * from './message-pane'
+export * from './message-pane-of.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane-of.tsx b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane-of.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..f2b2180b68c8b053570966ebf164c10cfa806eba
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane-of.tsx
@@ -0,0 +1,85 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type { Kit } from '../../kit'
+import Layout from '../../layout'
+import { FOOT_ROWS } from './foot-rows'
+import type { MessagePane } from './message-pane'
+
+/**
+ * The pane while a message stands in for the list (ReplDiffSidebar's
+ * centered body): the header, a blank row, the message, the pickers.
+ *
+ * With nothing listed below and room, the lines sit where the built-in
+ * centers them (half the spare rows above), the pickers under them;
+ * otherwise all stacks from the top, the pickers over the pre-session line.
+ *
+ * @param kit the elements, the width, the body's rows
+ * @param pane the header block, the message's lines, the pickers, the
+ *   pre-session line, and what is listed under it
+ * @returns the pane's tree
+ */
+export function messagePaneOf(kit: Kit, pane: MessagePane): RenderElement {
+  const { Box, Text } = kit.ui
+  const { top, controls, earlier, rest } = pane
+
+  const lines = pane.message.flatMap(text =>
+    Layout.wrappedLines(Layout.sanitizeName(text), kit.columns),
+  )
+
+  const drawn = lines.map(line => <Text dimColor>{line}</Text>)
+  const controlRows = controls === null ? [] : [controls]
+  const earlierRows = earlier === null ? [] : [earlier]
+
+  const isControlsAboveEarlier =
+    controlRows.length > 0 || earlierRows.length === 0
+
+  const aboveEarlier = isControlsAboveEarlier
+    ? controlRows
+    : [<Box height={1} />]
+
+  const footRows = earlier === null ? 0 : FOOT_ROWS
+  const middleRows = kit.rows - top.length - 1 - footRows
+  const above = Math.floor((middleRows - lines.length) / 2)
+
+  const isCentered =
+    rest.length === 0 && middleRows - above - lines.length >= controlRows.length
+
+  if (!isCentered) {
+    return (
+      <Box flexDirection="column">
+        {[
+          ...top,
+          <Box height={1} />,
+          ...drawn,
+          ...aboveEarlier,
+          ...earlierRows,
+          ...rest,
+        ]}
+      </Box>
+    )
+  }
+
+  const spacer = above > 0 ? [<Box height={above} />] : []
+
+  const foot = earlierRows.map(row => (
+    <Box flexDirection="column" marginTop={1} paddingBottom={1}>
+      {row}
+    </Box>
+  ))
+
+  return (
+    <Box flexDirection="column" height={kit.rows}>
+      {[
+        ...top,
+        <Box height={1} />,
+        <Box flexDirection="column" height={middleRows} alignItems="center">
+          {[...spacer, ...drawn, ...controlRows]}
+        </Box>,
+        ...foot,
+      ]}
+    </Box>
+  )
+}
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/index.ts b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..6e64d47d274514bb6f166c7e821e0b3338d0da18
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/index.ts
@@ -0,0 +1,3 @@
+export type * from './message-pane.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/message-pane.ts b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/message-pane.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9d42adcd42b302a75eae76aa85bb7ac26a44d6e3
--- /dev/null
+++ b/mods/diff/hooks/views/dialog-pane/message-pane-of/message-pane/message-pane.ts
@@ -0,0 +1,13 @@
+import type { RenderElement } from 'claude-code'
+
+/**
+ * What stands around a message in the pane: the header block, the message,
+ * the pickers, the pre-session line, and what is listed under it.
+ */
+export type MessagePane = {
+  top: readonly RenderElement[]
+  message: readonly string[]
+  controls: RenderElement | null
+  earlier: RenderElement | null
+  rest: readonly RenderElement[]
+}
diff --git a/mods/diff/hooks/views/entries/detail-model-of/detail-model-of.ts b/mods/diff/hooks/views/entries/detail-model-of/detail-model-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f6f21bf6c29c96c050e65a7c749c7fc6305650b2
--- /dev/null
+++ b/mods/diff/hooks/views/entries/detail-model-of/detail-model-of.ts
@@ -0,0 +1,24 @@
+import type PaneState from '../../../pane-state'
+import type Detail from '../../detail'
+import type { BodyEntry } from '../body-entry'
+
+/**
+ * A listed file as its detail draws it: the entry, whether it is the armed
+ * one, and the backend's words for its notes.
+ *
+ * @param entry the file
+ * @param model which file is armed, and the backend's words
+ * @returns the detail model
+ */
+export const detailModelOf = (
+  entry: BodyEntry,
+  model: Pick<PaneState.PaneModel, 'armedPath' | 'words'>,
+): Detail.DetailModel => ({
+  words: model.words,
+  path: entry.path,
+  displayPath: entry.displayPath,
+  isUntracked: entry.isUntracked,
+  isBinary: entry.isBinary,
+  body: entry.body,
+  isArmed: model.armedPath === entry.path,
+})
diff --git a/mods/diff/hooks/views/sections/ask-key-of/ask-key-of.ts b/mods/diff/hooks/views/sections/ask-key-of/ask-key-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b8354567305f1061b2430cad2ac2ce8b03c61cc7
--- /dev/null
+++ b/mods/diff/hooks/views/sections/ask-key-of/ask-key-of.ts
@@ -0,0 +1,10 @@
+import Layout from '../../layout'
+
+/**
+ * A file's ask Button address: the path made printable, under `ask:`, so
+ * each file's Button in the body has its own.
+ *
+ * @param path the file's path as git or the transcript gave it
+ * @returns the key
+ */
+export const askKeyOf = (path: string) => `ask:${Layout.sanitizeName(path)}`
diff --git a/mods/diff/hooks/views/sections/ask-key-of/index.ts b/mods/diff/hooks/views/sections/ask-key-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7d6b428c0434e9db57f7994cd291e0e0e5c637d1
--- /dev/null
+++ b/mods/diff/hooks/views/sections/ask-key-of/index.ts
@@ -0,0 +1,3 @@
+export * from './ask-key-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/dialog-file-row/dialog-file-row.tsx b/mods/diff/hooks/views/sections/dialog-file-row/dialog-file-row.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..ea0eefa982490247594db87e761fb0a496179330
--- /dev/null
+++ b/mods/diff/hooks/views/sections/dialog-file-row/dialog-file-row.tsx
@@ -0,0 +1,58 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import Limits from '../../../limits'
+import type { Kit } from '../../kit'
+import Layout from '../../layout'
+import { diffStat } from '../diff-stat'
+import type { FileRowModel } from '../file-row-model'
+import { listRow } from '../list-row'
+import { POINTER } from '../pointer'
+
+/**
+ * One file of the inline list (DiffFileList FileItem): the pointer before
+ * the selected one, where the focus ring starts, the name, counts or note.
+ *
+ * @param kit the elements, the handlers, the width
+ * @param row the file
+ * @param onPress selects the file
+ * @returns the row element
+ */
+export function dialogFileRow(
+  kit: Kit,
+  row: FileRowModel,
+  onPress: () => void,
+): RenderElement {
+  const { Text } = kit.ui
+
+  const room = Math.max(
+    Limits.DIALOG_PATH_FLOOR,
+    kit.columns - Limits.DIALOG_PATH_RESERVE,
+  )
+
+  const mark = row.isSelected ? POINTER : ' '
+  const name = Layout.truncateStart(Layout.sanitizeName(row.displayPath), room)
+
+  const note = (
+    <Text dimColor italic>
+      {row.note ?? ''}
+    </Text>
+  )
+
+  const hasNote = row.note !== null
+  const tail = hasNote ? note : diffStat(kit, row.added, row.removed)
+
+  return listRow(
+    kit,
+    {
+      key: row.key,
+      label: `${mark} ${name}`,
+      onPress,
+      isDim: false,
+      isAutoFocus: row.isSelected,
+    },
+    tail,
+  )
+}
diff --git a/mods/diff/hooks/views/sections/dialog-file-row/index.ts b/mods/diff/hooks/views/sections/dialog-file-row/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4e60674b3423f003e4a6ed6f70562a96eab8af98
--- /dev/null
+++ b/mods/diff/hooks/views/sections/dialog-file-row/index.ts
@@ -0,0 +1,3 @@
+export * from './dialog-file-row.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/diff-stat/diff-stat.tsx b/mods/diff/hooks/views/sections/diff-stat/diff-stat.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..423927e911b23c93d5c397a3013af990dd4be12f
--- /dev/null
+++ b/mods/diff/hooks/views/sections/diff-stat/diff-stat.tsx
@@ -0,0 +1,32 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type { Kit } from '../../kit'
+
+/**
+ * `+N -M` in the diff's word colours, a side left out at zero and nothing
+ * at all at 0/0 (design-system DiffStat).
+ *
+ * @param kit the drawing's kit; its elements draw the counts
+ * @param added lines added
+ * @param removed lines removed
+ * @returns the inline element
+ */
+export function diffStat(
+  kit: Kit,
+  added: number,
+  removed: number,
+): RenderElement {
+  const { Text } = kit.ui
+  const hasBoth = added > 0 && removed > 0
+
+  return (
+    <Text>
+      {added > 0 ? <Text color="diffAddedWord">{`+${added}`}</Text> : ''}
+      {hasBoth ? ' ' : ''}
+      {removed > 0 ? <Text color="diffRemovedWord">{`-${removed}`}</Text> : ''}
+    </Text>
+  )
+}
diff --git a/mods/diff/hooks/views/sections/diff-stat/index.ts b/mods/diff/hooks/views/sections/diff-stat/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..afaeb9c57bda991763deb504ea4fa4c184c923fb
--- /dev/null
+++ b/mods/diff/hooks/views/sections/diff-stat/index.ts
@@ -0,0 +1,3 @@
+export * from './diff-stat.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/dim-note/dim-note.tsx b/mods/diff/hooks/views/sections/dim-note/dim-note.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..0994309d75f2da51ba6e8bf4f20d6c4682c62571
--- /dev/null
+++ b/mods/diff/hooks/views/sections/dim-note/dim-note.tsx
@@ -0,0 +1,23 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type { Kit } from '../../kit'
+
+/**
+ * One dim line of the pane: a hint, an elision count, a base label.
+ *
+ * @param kit the drawing's kit; its elements draw the line
+ * @param text the line
+ * @returns the element
+ */
+export function dimNote(kit: Kit, text: string): RenderElement {
+  const { Text } = kit.ui
+
+  return (
+    <Text dimColor wrap="truncate-end">
+      {text}
+    </Text>
+  )
+}
diff --git a/mods/diff/hooks/views/sections/dim-note/index.ts b/mods/diff/hooks/views/sections/dim-note/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7adf3e29e8960e8ad23a316cccdc17a126eb9611
--- /dev/null
+++ b/mods/diff/hooks/views/sections/dim-note/index.ts
@@ -0,0 +1,3 @@
+export * from './dim-note.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/file-key-of/file-key-of.ts b/mods/diff/hooks/views/sections/file-key-of/file-key-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c917b6680760f87e7c01f0280eb7258ea6b023e8
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-key-of/file-key-of.ts
@@ -0,0 +1,11 @@
+import type { BodyEntry } from '../../entries'
+import Layout from '../../layout'
+
+/**
+ * A file row's Button address: the path made printable, under `file:`.
+ *
+ * @param entry the file, its path as git or the transcript gave it
+ * @returns the key
+ */
+export const fileKeyOf = (entry: Pick<BodyEntry, 'path'>) =>
+  `file:${Layout.sanitizeName(entry.path)}`
diff --git a/mods/diff/hooks/views/sections/file-key-of/index.ts b/mods/diff/hooks/views/sections/file-key-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c7db4eb43595a52900b5f31c2c149acf9356f60e
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-key-of/index.ts
@@ -0,0 +1,3 @@
+export * from './file-key-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/file-row-model/file-row-model.ts b/mods/diff/hooks/views/sections/file-row-model/file-row-model.ts
new file mode 100644
index 0000000000000000000000000000000000000000..482a71cc7107238133bcaf732dccbeb897325263
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-row-model/file-row-model.ts
@@ -0,0 +1,13 @@
+/**
+ * One listed file as its row draws it: the Button's address, the path and
+ * the name listed, the counts or (inline) a dim note for them, selection.
+ */
+export type FileRowModel = {
+  key: string
+  path: string
+  displayPath: string
+  added: number
+  removed: number
+  note: string | null
+  isSelected: boolean
+}
diff --git a/mods/diff/hooks/views/sections/file-row-model/index.ts b/mods/diff/hooks/views/sections/file-row-model/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9bac45bae3de313dfa17c0347832cbe5b3e5f7a1
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-row-model/index.ts
@@ -0,0 +1,3 @@
+export type * from './file-row-model.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/file-row/file-row.ts b/mods/diff/hooks/views/sections/file-row/file-row.ts
new file mode 100644
index 0000000000000000000000000000000000000000..afb792c04e80c6d76522d962a22baa6be38c1bf1
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-row/file-row.ts
@@ -0,0 +1,41 @@
+import type { RenderElement } from 'claude-code'
+
+import Limits from '../../../limits'
+import type { Kit } from '../../kit'
+import Layout from '../../layout'
+import { diffStat } from '../diff-stat'
+import type { FileRowModel } from '../file-row-model'
+import { listRow } from '../list-row'
+import { statTextOf } from '../stat-text-of'
+
+/**
+ * One file of the docked list (ReplDiffSidebar FileSummaryRow): the
+ * start-truncated name as a plain Button, its counts at the right.
+ *
+ * Nothing at the right at 0/0 (an untracked or binary file). The label is
+ * padded up to the counts, so the whole row before them presses.
+ *
+ * @param kit the elements, the handlers, the width
+ * @param row the file
+ * @param onPress scrolls the body to the file
+ * @returns the row element
+ */
+export function fileRow(
+  kit: Kit,
+  row: FileRowModel,
+  onPress: () => void,
+): RenderElement {
+  const label = Layout.padEnd(
+    Layout.truncateStart(
+      Layout.sanitizeName(row.displayPath),
+      Math.max(kit.columns - Limits.STAT_CELLS, Limits.PATH_FLOOR),
+    ),
+    kit.columns - statTextOf(row.added, row.removed).length,
+  )
+
+  return listRow(
+    kit,
+    { key: row.key, label, onPress, isDim: true, isAutoFocus: false },
+    diffStat(kit, row.added, row.removed),
+  )
+}
diff --git a/mods/diff/hooks/views/sections/file-row/index.ts b/mods/diff/hooks/views/sections/file-row/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1394b6f679ef41984a7dea216f2c621142cf5e42
--- /dev/null
+++ b/mods/diff/hooks/views/sections/file-row/index.ts
@@ -0,0 +1,3 @@
+export * from './file-row.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/keyed-pressable/index.ts b/mods/diff/hooks/views/sections/keyed-pressable/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ddcdd6dd750d4986513433637d45cbe633a748ec
--- /dev/null
+++ b/mods/diff/hooks/views/sections/keyed-pressable/index.ts
@@ -0,0 +1,3 @@
+export type * from './keyed-pressable.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/keyed-pressable/keyed-pressable.ts b/mods/diff/hooks/views/sections/keyed-pressable/keyed-pressable.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7725f8d5669e05156454a863d42051768c022434
--- /dev/null
+++ b/mods/diff/hooks/views/sections/keyed-pressable/keyed-pressable.ts
@@ -0,0 +1,11 @@
+import type { Pressable } from '../pressable'
+
+/**
+ * A plain Button's face, press and address: a list row's Button, dim at
+ * rest where the built-in's is, the focus ring's start where selected.
+ */
+export type KeyedPressable = Pressable & {
+  key: string
+  isDim: boolean
+  isAutoFocus: boolean
+}
diff --git a/mods/diff/hooks/views/sections/list-edge-button/index.ts b/mods/diff/hooks/views/sections/list-edge-button/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..99509f2282da5fdcde8e656f9f5a6feb7c28a87e
--- /dev/null
+++ b/mods/diff/hooks/views/sections/list-edge-button/index.ts
@@ -0,0 +1,3 @@
+export * from './list-edge-button.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/list-edge-button/list-edge-button.tsx b/mods/diff/hooks/views/sections/list-edge-button/list-edge-button.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..6840f0fc69859e4f928b6c6eb9664c9bc681712e
--- /dev/null
+++ b/mods/diff/hooks/views/sections/list-edge-button/list-edge-button.tsx
@@ -0,0 +1,36 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type { Kit } from '../../kit'
+
+/**
+ * The docked list's `more above` or `more below` row as a dim plain Button
+ * that moves its window a file, under the built-in's list keys too.
+ *
+ * @param kit the elements and the handlers
+ * @param edge which edge, the Button's key
+ * @param label the row's text
+ * @returns the Button
+ */
+export function listEdgeButton(
+  kit: Kit,
+  edge: 'list-up' | 'list-down',
+  label: string,
+): RenderElement {
+  const { Button } = kit.ui
+  const isUp = edge === 'list-up'
+
+  return (
+    <Button
+      key={edge}
+      plain
+      dimColor
+      action={isUp ? 'app:diffFileListUp' : 'app:diffFileListDown'}
+      onPress={() => kit.actions.scrollList(isUp ? -1 : 1)}
+    >
+      {label}
+    </Button>
+  )
+}
diff --git a/mods/diff/hooks/views/sections/list-row/index.ts b/mods/diff/hooks/views/sections/list-row/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0bb830e59ff68ca484597e8fbfe5df7b9ee53549
--- /dev/null
+++ b/mods/diff/hooks/views/sections/list-row/index.ts
@@ -0,0 +1,3 @@
+export * from './list-row.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/list-row/list-row.tsx b/mods/diff/hooks/views/sections/list-row/list-row.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..3a50414cf31f82bb757e6b373a3e9a9c6470eaf1
--- /dev/null
+++ b/mods/diff/hooks/views/sections/list-row/list-row.tsx
@@ -0,0 +1,40 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type { Kit } from '../../kit'
+import type { KeyedPressable } from '../keyed-pressable'
+
+/**
+ * One list row: a plain Button as wide as its label, a spacer, then the
+ * tail at the right edge (the counts, or a note).
+ *
+ * @param kit the elements
+ * @param row the Button's key, label, press, rest style and ring start
+ * @param tail what sits at the row's right edge
+ * @returns the row element
+ */
+export function listRow(
+  kit: Kit,
+  row: KeyedPressable,
+  tail: RenderElement,
+): RenderElement {
+  const { Box, Button } = kit.ui
+
+  return (
+    <Box flexDirection="row">
+      <Button
+        key={row.key}
+        plain
+        dimColor={row.isDim}
+        {...(row.isAutoFocus ? ({ autoFocus: true } as const) : {})}
+        onPress={row.onPress}
+      >
+        {row.label}
+      </Button>
+      <Box flexGrow={1} />
+      {tail}
+    </Box>
+  )
+}
diff --git a/mods/diff/hooks/views/sections/name-row/index.ts b/mods/diff/hooks/views/sections/name-row/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3daf5106881f2b115018b04dd2bd22ecfc081e11
--- /dev/null
+++ b/mods/diff/hooks/views/sections/name-row/index.ts
@@ -0,0 +1,3 @@
+export * from './name-row.jsx'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/name-row/name-row.tsx b/mods/diff/hooks/views/sections/name-row/name-row.tsx
new file mode 100644
index 0000000000000000000000000000000000000000..af64d727edf21444501752a6c6dceb383cb8af54
--- /dev/null
+++ b/mods/diff/hooks/views/sections/name-row/name-row.tsx
@@ -0,0 +1,56 @@
+/* @jsxRuntime classic */
+/* @jsx h */
+/* @jsxFrag Fragment */
+import type { RenderElement } from 'claude-code'
+
+import type Detail from '../../detail'
+import type { Kit } from '../../kit'
+import Layout from '../../layout'
+import { askKeyOf } from '../ask-key-of'
+
+/**
+ * A file's name row over its body (DiffDetailView's first line): the bold
+ * name cut from its start, its asides in dim, and its ask Button.
+ *
+ * The Button reads `asked ✓` while the file is armed; a file with no hunks
+ * to attach (a placeholder body) has none.
+ *
+ * @param kit the elements, the handlers, the width
+ * @param name the file, with whether it is truncated, armed and askable
+ * @returns the row
+ */
+export function nameRow(kit: Kit, name: Detail.NamedDetail): RenderElement {
+  const { Box, Text, Button } = kit.ui
+
+  const asides = [
+    name.isUntracked ? 'untracked' : null,
+    name.isTruncated ? 'truncated' : null,
+  ].filter(word => word !== null)
+
+  const aside = asides.length === 0 ? '' : ` (${asides.join(', ')})`
+
+  const ask = (
+    <Button
+      key={askKeyOf(name.path)}
+      onPress={() => kit.actions.toggleAsk(name.path)}
+    >
+      {name.isArmed ? 'asked ✓' : 'ask'}
+    </Button>
+  )
+
+  return (
+    <Box flexDirection="row">
+      {[
+        <Text bold wrap="truncate-start">
+          {Layout.truncateStart(
+            Layout.sanitizeName(name.displayPath),
+            kit.columns,
+          )}
+        </Text>,
+        <Text dimColor>{aside}</Text>,
+        <Box flexGrow={1} />,
+        ...(name.isAskable ? [ask] : []),
+      ]}
+    </Box>
+  )
+}
diff --git a/mods/diff/hooks/views/sections/pointer/index.ts b/mods/diff/hooks/views/sections/pointer/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..eb89af57ff0454d801ae649d5869aad96953175f
--- /dev/null
+++ b/mods/diff/hooks/views/sections/pointer/index.ts
@@ -0,0 +1,3 @@
+export * from './pointer.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/pointer/pointer.ts b/mods/diff/hooks/views/sections/pointer/pointer.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d0e13e25f92cb71ca4b6d4ccb9230127f6374658
--- /dev/null
+++ b/mods/diff/hooks/views/sections/pointer/pointer.ts
@@ -0,0 +1,4 @@
+/**
+ * Drawn before the selected file's row (DiffFileList's pointer).
+ */
+export const POINTER = '❯'
diff --git a/mods/diff/hooks/views/sections/pressable/index.ts b/mods/diff/hooks/views/sections/pressable/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8d5fc84f834f24d1d3f0d74dadfb4c42f141fe65
--- /dev/null
+++ b/mods/diff/hooks/views/sections/pressable/index.ts
@@ -0,0 +1,3 @@
+export type * from './pressable.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/pressable/pressable.ts b/mods/diff/hooks/views/sections/pressable/pressable.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3585854fe48c72c51070ea2a2e44aee22df5db79
--- /dev/null
+++ b/mods/diff/hooks/views/sections/pressable/pressable.ts
@@ -0,0 +1,11 @@
+/**
+ * A plain Button's face and what pressing it does.
+ */
+export type Pressable = {
+  label: string
+
+  /**
+   * Runs on a click or Enter.
+   */
+  onPress: () => void
+}
diff --git a/mods/diff/hooks/views/sections/stat-text-of/index.ts b/mods/diff/hooks/views/sections/stat-text-of/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..11204da21eaf56271d97d348a1453f93e995f690
--- /dev/null
+++ b/mods/diff/hooks/views/sections/stat-text-of/index.ts
@@ -0,0 +1,3 @@
+export * from './stat-text-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/views/sections/stat-text-of/stat-text-of.ts b/mods/diff/hooks/views/sections/stat-text-of/stat-text-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..a5ff21ff2715307ec1b20d10117109c9682500ec
--- /dev/null
+++ b/mods/diff/hooks/views/sections/stat-text-of/stat-text-of.ts
@@ -0,0 +1,15 @@
+import { keptOf } from '../../../kept-of'
+
+/**
+ * `+N -M` as diffStat draws it, as plain text: what a row measures to know
+ * where its counts start; empty at 0/0.
+ *
+ * @param added lines added
+ * @param removed lines removed
+ * @returns the text
+ */
+export const statTextOf = (added: number, removed: number) =>
+  keptOf([
+    added > 0 ? `+${added}` : null,
+    removed > 0 ? `-${removed}` : null,
+  ]).join(' ')
diff --git a/mods/diff/tests/ask/fitted-ask-text-of.test.ts b/mods/diff/tests/ask/fitted-ask-text-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f98c6f48ec0db0382817753069b1b924d2682b66
--- /dev/null
+++ b/mods/diff/tests/ask/fitted-ask-text-of.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Ask from '../../hooks/ask'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('fitted-ask-text-of', () => {
+  test('an ask that fits rides whole', () => {
+    expect(
+      Ask.fittedAskTextOf(Fixtures.ASK_TEXT, Fixtures.ASK_TEXT.length),
+    ).toBe(Fixtures.ASK_TEXT)
+  })
+
+  test('an ask past the room keeps its first lines and the cut note', () => {
+    const fitted = Ask.fittedAskTextOf(
+      Fixtures.ASK_TEXT,
+      Fixtures.ROOM_FOR_TWO_LINES,
+    )
+
+    expect(fitted).toBe(
+      `${Fixtures.ASK_LINES.slice(0, 2).join('\n')}\n${Ask.ASK_CUT_NOTE}`,
+    )
+
+    expect(fitted?.length).toBeLessThanOrEqual(Fixtures.ROOM_FOR_TWO_LINES)
+  })
+
+  test('an ask whose first hunk line does not fit is not attached', () => {
+    expect(
+      Ask.fittedAskTextOf(Fixtures.ASK_TEXT, Fixtures.ROOM_FOR_ONE_LINE),
+    ).toBeUndefined()
+  })
+})
diff --git a/mods/diff/tests/backend/backend-of.test.ts b/mods/diff/tests/backend/backend-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..8103c7d1b7d20088949c1c67f28c6b1262d7c1e1
--- /dev/null
+++ b/mods/diff/tests/backend/backend-of.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Backend from '../../hooks/backend'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('backend-of', () => {
+  test('as published: no probes, so git or nothing', async () => {
+    const inGit = Fixtures.gitHostOf()
+    const outside = Fixtures.recordingBackendHostOf({})
+
+    expect((await Backend.backendOf(inGit.host, []))?.words.lister).toBe('git')
+    expect(await Backend.backendOf(outside.host, [])).toBeNull()
+
+    expect(
+      [inGit.programs, outside.programs],
+      'pinning spawns the one rev-parse and walks nothing',
+    ).toEqual([['git'], ['git']])
+  })
+
+  test('a declining probe leaves a git working tree to git', async () => {
+    const declining = Fixtures.recordingProbeOf(null)
+    const inGit = Fixtures.gitHostOf()
+    const backend = await Backend.backendOf(inGit.host, [declining.probe])
+
+    expect(backend?.words.lister).toBe('git')
+    expect(backend?.baseModes).toEqual(['session', 'uncommitted', 'branch'])
+    expect(declining.asked).toEqual([inGit.host])
+    expect(inGit.programs).toEqual(['git'])
+  })
+
+  test('a probe that answers pins before git is asked', async () => {
+    const inGit = Fixtures.gitHostOf()
+
+    const backend = await Backend.backendOf(inGit.host, [
+      Fixtures.recordingProbeOf('/home/me/src').probe,
+    ])
+
+    expect(backend?.repository).toEqual({ toplevel: '/home/me/src' })
+    expect(inGit.programs).toEqual([])
+  })
+
+  test('the probes are asked in order and the first answer pins', async () => {
+    const declining = Fixtures.recordingProbeOf(null)
+    const first = Fixtures.recordingProbeOf('/first')
+    const second = Fixtures.recordingProbeOf('/second')
+    const { host } = Fixtures.recordingBackendHostOf({})
+
+    const backend = await Backend.backendOf(host, [
+      declining.probe,
+      first.probe,
+      second.probe,
+    ])
+
+    expect(backend?.repository).toEqual({ toplevel: '/first' })
+
+    expect([
+      declining.asked.length,
+      first.asked.length,
+      second.asked.length,
+    ]).toEqual([1, 1, 0])
+
+    expect(declining.asked[0]).toBe(host)
+  })
+})
diff --git a/mods/diff/tests/classify/is-noise-file.test.ts b/mods/diff/tests/classify/is-noise-file.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4a1fd54f5eaa17f5290ece839df227c7c2cdb91a
--- /dev/null
+++ b/mods/diff/tests/classify/is-noise-file.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Classify from '../../hooks/classify'
+
+tier('builtin')
+
+describe('is-noise-file', () => {
+  test('tests, fixtures and snapshots are noise', () => {
+    for (const path of [
+      'test/a.ts',
+      'src/x/__tests__/y.ts',
+      'lib/foo.test.tsx',
+      'pkg/bar_test.go',
+      'web/fixtures/data.json',
+      'ui/__snapshots__/a.snap',
+    ]) {
+      expect(Classify.isNoiseFile(path), path).toBe(true)
+    }
+  })
+
+  test('lockfiles, builds, minified and generated files are noise', () => {
+    for (const path of [
+      'bun.lock',
+      'package-lock.json',
+      'dist/index.js',
+      'assets/app.min.js',
+      'types/api.d.ts',
+      'src/constants/prompts.generated/x.ts',
+      'proto/service.pb.go',
+    ]) {
+      expect(Classify.isNoiseFile(path), path).toBe(true)
+    }
+  })
+
+  test('source is not, even with test inside a longer word', () => {
+    for (const path of ['src/contest/entry.ts', 'src/latest.ts', 'README.md']) {
+      expect(Classify.isNoiseFile(path), path).toBe(false)
+    }
+  })
+})
diff --git a/mods/diff/tests/fixtures/answers-of.ts b/mods/diff/tests/fixtures/answers-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d5ba319c5893d3521597e3cf47f4e73491c2033c
--- /dev/null
+++ b/mods/diff/tests/fixtures/answers-of.ts
@@ -0,0 +1,27 @@
+/**
+ * Git's output in /work for each invocation whose command line holds the
+ * key, over the given `--numstat -z` records and per-file bodies.
+ *
+ * `status` lists the paths dirty at the start (the baseline: every path
+ * with a body unless given); a body is keyed by the `-- <path>` its diff is
+ * asked with.
+ *
+ * @param numstat the `--numstat -z` records, joined
+ * @param bodies each path's unified diff after its header
+ * @param dirty the paths `status` lists as dirty when the session starts
+ * @returns the answers by key
+ */
+export const answersOf = (
+  numstat: string,
+  bodies: Readonly<Record<string, string>>,
+  dirty: readonly string[] = Object.keys(bodies),
+): Readonly<Record<string, string>> => ({
+  'rev-parse --path-format=absolute': '/work\n/work/.git\n/work/.git\n',
+  'status --porcelain': dirty.map(path => ` M ${path}\0`).join(''),
+  'HEAD --shortstat': ` ${Object.keys(bodies).length} files changed`,
+  'HEAD --numstat': numstat,
+  'ls-files': '',
+  ...Object.fromEntries(
+    Object.entries(bodies).map(([path, body]) => [`-- ${path}`, body]),
+  ),
+})
diff --git a/mods/diff/tests/fixtures/big-lines.ts b/mods/diff/tests/fixtures/big-lines.ts
new file mode 100644
index 0000000000000000000000000000000000000000..ef3e047ce74e9b182c1a531aa817d868f580ed3f
--- /dev/null
+++ b/mods/diff/tests/fixtures/big-lines.ts
@@ -0,0 +1,4 @@
+/**
+ * A hunk length past what one turn's diff keeps whole.
+ */
+export const BIG_LINES = 1000
diff --git a/mods/diff/tests/fixtures/builtin-holds.ts b/mods/diff/tests/fixtures/builtin-holds.ts
new file mode 100644
index 0000000000000000000000000000000000000000..58f081c8e98c13cf3fa3a3934714661d2b144250
--- /dev/null
+++ b/mods/diff/tests/fixtures/builtin-holds.ts
@@ -0,0 +1,4 @@
+/**
+ * How `$.command.register` refuses `/diff` while the built-in one is listed.
+ */
+export const BUILTIN_HOLDS = '"/diff" refused: it is the built-in /diff'
diff --git a/mods/diff/tests/fixtures/checkout.ts b/mods/diff/tests/fixtures/checkout.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d6e0c6c01a8ef9affcc60207404ae6ced374dd3b
--- /dev/null
+++ b/mods/diff/tests/fixtures/checkout.ts
@@ -0,0 +1,11 @@
+import type Git from '../../hooks/git'
+
+/**
+ * Where every model and view fixture sits: a plain checkout at `/r` whose
+ * git dir is its common dir.
+ */
+export const CHECKOUT: Git.Repository = {
+  toplevel: '/r',
+  gitDir: '/r/.git',
+  commonDir: '/r/.git',
+}
diff --git a/mods/diff/tests/fixtures/clear.ts b/mods/diff/tests/fixtures/clear.ts
new file mode 100644
index 0000000000000000000000000000000000000000..05ffd4f8a3a7021a4df1ad448ce611df4275a0bb
--- /dev/null
+++ b/mods/diff/tests/fixtures/clear.ts
@@ -0,0 +1,11 @@
+import type { CommandRunInput } from 'claude-code'
+
+/**
+ * The command that starts the conversation over, as the person types it.
+ */
+export const CLEAR: CommandRunInput = {
+  command: 'clear',
+  args: '',
+  origin: { kind: 'composer' },
+  presentation: { isFullscreen: true, columns: 160 },
+}
diff --git a/mods/diff/tests/fixtures/dialog-diff.ts b/mods/diff/tests/fixtures/dialog-diff.ts
new file mode 100644
index 0000000000000000000000000000000000000000..780d06cda2bea2d21ebb5a4bd257a54e9e4dfb54
--- /dev/null
+++ b/mods/diff/tests/fixtures/dialog-diff.ts
@@ -0,0 +1,12 @@
+import type { CommandRunInput } from 'claude-code'
+
+import { DIFF } from './diff.js'
+
+/**
+ * The command as the person types it on the main screen (no fullscreen
+ * layout), where the built-in answers with its dialog.
+ */
+export const DIALOG_DIFF: CommandRunInput = {
+  ...DIFF,
+  presentation: { isFullscreen: false, columns: 100 },
+}
diff --git a/mods/diff/tests/fixtures/diff-at.ts b/mods/diff/tests/fixtures/diff-at.ts
new file mode 100644
index 0000000000000000000000000000000000000000..43d8bff5c2c1106f3be5ccd61795ac2809d4bdca
--- /dev/null
+++ b/mods/diff/tests/fixtures/diff-at.ts
@@ -0,0 +1,15 @@
+import type { CommandRunInput } from 'claude-code'
+
+import { DIFF } from './diff.js'
+
+/**
+ * The command typed under the fullscreen layout on a terminal so many
+ * columns wide, as its presentation tells the hooks.
+ *
+ * @param columns the terminal's width
+ * @returns the command's input
+ */
+export const diffAt = (columns: number): CommandRunInput => ({
+  ...DIFF,
+  presentation: { isFullscreen: true, columns },
+})
diff --git a/mods/diff/tests/fixtures/diff.ts b/mods/diff/tests/fixtures/diff.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3c9eb41ea531445ff038a2fc0a3564c0641374b4
--- /dev/null
+++ b/mods/diff/tests/fixtures/diff.ts
@@ -0,0 +1,12 @@
+import type { CommandRunInput } from 'claude-code'
+
+/**
+ * The command as the person types it, with no arguments, in the fullscreen
+ * layout on a 160-column terminal.
+ */
+export const DIFF: CommandRunInput = {
+  command: 'diff',
+  args: '',
+  origin: { kind: 'composer' },
+  presentation: { isFullscreen: true, columns: 160 },
+}
diff --git a/mods/diff/tests/fixtures/edit-patch-of.ts b/mods/diff/tests/fixtures/edit-patch-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..b6fff5a156a0f898e7aebfeb45c23ee26e2257af
--- /dev/null
+++ b/mods/diff/tests/fixtures/edit-patch-of.ts
@@ -0,0 +1,12 @@
+/**
+ * An Edit tool's result record holding one hunk of one file.
+ *
+ * @param lines the hunk's lines, `-`/`+`/space prefixed
+ * @returns the record as the transcript stores it
+ */
+export const editPatchOf = (lines: string[]) => ({
+  filePath: '/r/a.ts',
+  structuredPatch: [
+    { oldStart: 1, oldLines: 1, newStart: 1, newLines: 1, lines },
+  ],
+})
diff --git a/mods/diff/tests/fixtures/git-hung.ts b/mods/diff/tests/fixtures/git-hung.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e2c27a595e75faefe5dd859b8e7c93a857dbd7b5
--- /dev/null
+++ b/mods/diff/tests/fixtures/git-hung.ts
@@ -0,0 +1,7 @@
+import Limits from '../../hooks/limits'
+
+/**
+ * How `$.process.run` refuses a git that outran its timeout.
+ */
+export const GIT_HUNG =
+  'git aborted: still running after ' + `${Limits.GIT_TIMEOUT_MS}ms`
diff --git a/mods/diff/tests/fixtures/git-in.ts b/mods/diff/tests/fixtures/git-in.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1a63b69fae43c3e753cdb71f41d96c09d33b3428
--- /dev/null
+++ b/mods/diff/tests/fixtures/git-in.ts
@@ -0,0 +1,27 @@
+import type { ProcessRunResult } from 'claude-code'
+
+import { NOT_A_REPOSITORY } from './not-a-repository.js'
+import { REPOSITORY } from './repository.js'
+
+/**
+ * What git answers from a script of outputs by command-line key, REPOSITORY
+ * when none is given.
+ *
+ * An invocation the script does not know fails as git does outside a
+ * repository.
+ *
+ * @param argv the invocation, program first
+ * @param script git's output for each invocation whose line holds the key
+ * @returns git's exit code and output
+ */
+export function gitIn(
+  argv: readonly string[],
+  script: Readonly<Record<string, string>> = REPOSITORY,
+): ProcessRunResult {
+  const line = argv.join(' ')
+  const found = Object.entries(script).find(([key]) => line.includes(key))
+
+  return found
+    ? { exitCode: 0, stdout: found[1], stderr: '' }
+    : NOT_A_REPOSITORY
+}
diff --git a/mods/diff/tests/fixtures/git-word-of.ts b/mods/diff/tests/fixtures/git-word-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d8a06509afeaacc7af0a7f483c66873f546def10
--- /dev/null
+++ b/mods/diff/tests/fixtures/git-word-of.ts
@@ -0,0 +1,36 @@
+/**
+ * One git invocation named by what it asks: its subcommand, and its kind.
+ *
+ * For a `diff` what kind (`--shortstat`, `--numstat`, or the path its body
+ * is asked for), for a `rev-parse` whether it finds the repository
+ * (`--show-toplevel`) or reads `HEAD` (the poll's fallback where no file
+ * answers), so a test reads a session's spawns as the built-in's list.
+ *
+ * @param argv the invocation, program first, any pinned lead after it
+ * @returns `rev-parse --show-toplevel`, `status`, `diff --numstat`, ...
+ */
+export function gitWordOf(argv: readonly string[]) {
+  const [subcommand = '?'] = argv.filter(
+    (word, at) => at > 0 && !word.startsWith('-') && argv[at - 1] !== '-c',
+  )
+
+  const dashes = argv.indexOf('--')
+
+  if (subcommand === 'rev-parse') {
+    return argv.includes('--show-toplevel')
+      ? 'rev-parse --show-toplevel'
+      : 'rev-parse HEAD'
+  }
+
+  if (subcommand !== 'diff') {
+    return subcommand
+  }
+
+  if (dashes !== -1) {
+    return `diff -- ${argv.slice(dashes + 1).join(' ')}`
+  }
+
+  const stat = argv.find(word => word === '--shortstat' || word === '--numstat')
+
+  return stat ? `diff ${stat}` : 'diff'
+}
diff --git a/mods/diff/tests/fixtures/hint-at.ts b/mods/diff/tests/fixtures/hint-at.ts
new file mode 100644
index 0000000000000000000000000000000000000000..485a149ff462aace60b0fe2fb1b17c0272f00aae
--- /dev/null
+++ b/mods/diff/tests/fixtures/hint-at.ts
@@ -0,0 +1,19 @@
+import type { RenderInput } from 'claude-code'
+
+import { HINT } from './hint.js'
+
+/**
+ * The prompt's hint on a fullscreen terminal so many columns wide: drawing
+ * it is how the plugin learns the width.
+ *
+ * @param columns the terminal's width
+ * @returns the `ui.render` input
+ */
+export const hintAt = (columns: number): RenderInput<'PromptHint'> => ({
+  ...HINT,
+  viewport: {
+    columns,
+    rows: HINT.viewport?.rows ?? columns,
+    isFullscreen: true,
+  },
+})
diff --git a/mods/diff/tests/fixtures/hint-drawn.ts b/mods/diff/tests/fixtures/hint-drawn.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c9dd87da6abc93f9d1f4ca380f4b472c34eb108a
--- /dev/null
+++ b/mods/diff/tests/fixtures/hint-drawn.ts
@@ -0,0 +1,9 @@
+import type { RenderElement } from 'claude-code'
+
+/**
+ * What the engine draws for the hint, standing in beneath the plugin.
+ */
+export const HINT_DRAWN: RenderElement = {
+  type: 'Text',
+  children: ['? for shortcuts'],
+}
diff --git a/mods/diff/tests/fixtures/hint.ts b/mods/diff/tests/fixtures/hint.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1205ef8c4a19c6eb21f1c789799a6b48281e2d18
--- /dev/null
+++ b/mods/diff/tests/fixtures/hint.ts
@@ -0,0 +1,14 @@
+import type { RenderInput } from 'claude-code'
+
+/**
+ * The prompt's hint on a 160-column terminal under the fullscreen layout:
+ * drawing it is how the plugin learns how wide the terminal is and whether a
+ * pane docks beside the transcript there.
+ */
+export const HINT: RenderInput<'PromptHint'> = {
+  component: 'PromptHint',
+  surface: 'terminal',
+  requestId: 'hint',
+  viewport: { columns: 160, rows: 40, isFullscreen: true },
+  props: { isDraft: false, isWorking: false, hint: '' },
+}
diff --git a/mods/diff/tests/fixtures/in-repository.ts b/mods/diff/tests/fixtures/in-repository.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4310190166c2d6c404b5369e4c5c781ca1258bb3
--- /dev/null
+++ b/mods/diff/tests/fixtures/in-repository.ts
@@ -0,0 +1,69 @@
+import type { Args, On } from 'claude-code'
+import { mock } from 'claude-code/testing'
+
+import { gitIn } from './git-in.js'
+import { HINT_DRAWN } from './hint-drawn.js'
+import { keeping } from './keeping.js'
+import { REPOSITORY } from './repository.js'
+import { startsSession } from './starts-session.js'
+
+/**
+ * A session in a repository git answers for from a script (REPOSITORY, in
+ * /work, when none is given), keeping what the plugin does there.
+ *
+ * Kept: each git run, ring move, pane opened or closed, status line. The clock
+ * starts at 0 and the engine draws the hint. A test that rewrites the
+ * script between calls changes what git answers next.
+ *
+ * @param on the test's `on`
+ * @param script git's output for each invocation whose line holds the key
+ * @param stored what the plugin's store holds at the start
+ * @returns the runs, the ring's moves, the panes opened and closed, the
+ *   statuses, the clock
+ */
+export function inRepository(
+  on: On,
+  script: Readonly<Record<string, string>> = REPOSITORY,
+  stored: Readonly<Record<string, unknown>> = {},
+) {
+  const runs: Args<'process.run'>[] = []
+  const focused: Args<'ui.focus'>[] = []
+  const statuses: (string | undefined)[] = []
+  const opened = keeping<Args<'ui.open'>>()
+  const closed = keeping<Args<'ui.close'>>()
+  const clock = startsSession(on)
+
+  on('process.run', ($, e) => {
+    runs.push(e)
+
+    return { value: gitIn(e.argv, script) }
+  })
+
+  on('ui.focus', (_engine, e) => {
+    focused.push(e)
+
+    return {}
+  })
+
+  on('ui.status', ($, e) => {
+    statuses.push(e.text)
+
+    return { value: undefined }
+  })
+
+  on('ui.open', opened.hook)
+  on('ui.close', closed.hook)
+  on('ui.invalidate', () => ({ value: undefined }))
+  on('ui.render', { component: 'PromptHint' }, () => HINT_DRAWN)
+  on('session.messages', () => ({ value: [] }))
+  mock.store(on, stored)
+
+  return {
+    runs,
+    focused,
+    opened: opened.kept,
+    closed: closed.kept,
+    statuses,
+    clock,
+  }
+}
diff --git a/mods/diff/tests/fixtures/index.ts b/mods/diff/tests/fixtures/index.ts
new file mode 100644
index 0000000000000000000000000000000000000000..aa75150711ec11ccb9815eb034d681d53da5298b
--- /dev/null
+++ b/mods/diff/tests/fixtures/index.ts
@@ -0,0 +1,62 @@
+export * from './answers-of.js'
+export * from './ask'
+export * from './backend-of'
+export * from './big-lines.js'
+export * from './branch-base-of'
+export * from './builtin-holds.js'
+export * from './checkout.js'
+export * from './clear.js'
+export * from './code-blocks-of'
+export * from './datings-of'
+export * from './dialog-diff.js'
+export * from './diff.js'
+export * from './diff-at.js'
+export * from './edit-patch-of.js'
+export * from './fetch-diff'
+export * from './git-hung.js'
+export * from './git-in.js'
+export * from './git-word-of.js'
+export * from './head-key-of'
+export * from './header-totals-of'
+export * from './hint.js'
+export * from './hint-at.js'
+export * from './hint-drawn.js'
+export * from './in-repository.js'
+export * from './inline-pane.js'
+export * from './keeping.js'
+export * from './kind-walk-of'
+export * from './main-screen-hint.js'
+export * from './many-file-count.js'
+export * from './many-files.js'
+export * from './mode-label-of'
+export * from './moved-in.js'
+export * from './narrow-inline-pane.js'
+export * from './no-stats.js'
+export * from './not-a-repository.js'
+export * from './old-files.js'
+export * from './pane.js'
+export * from './pane-view'
+export * from './parse-file-diff'
+export * from './poll-word.js'
+export * from './renamed.js'
+export * from './repository.js'
+export * from './repository-of'
+export * from './ring-onto.js'
+export * from './scripted'
+export * from './session.js'
+export * from './settle-ms.js'
+export * from './starts-session.js'
+export * from './todo-write-of.js'
+export * from './tree'
+export * from './truncate-start'
+export * from './turn-diffs-of'
+export * from './two-files.js'
+export * from './typed-prompt-of.js'
+export * from './unsaid-hint.js'
+export * from './vs-main.js'
+export * from './wheel-over-list.js'
+export * from './wheel-tick.js'
+export * from './worktree'
+export * from './wrapped-lines'
+
+export * as default from '.'
diff --git a/mods/diff/tests/fixtures/inline-pane.ts b/mods/diff/tests/fixtures/inline-pane.ts
new file mode 100644
index 0000000000000000000000000000000000000000..7acab4a517c6de763f8ca26303ee6640d4c055d9
--- /dev/null
+++ b/mods/diff/tests/fixtures/inline-pane.ts
@@ -0,0 +1,12 @@
+import type { RenderInput } from 'claude-code'
+
+import { PANE } from './pane.js'
+
+/**
+ * The diff pane seated inline above the prompt on a 160-column terminal,
+ * as a session without the fullscreen layout seats it.
+ */
+export const INLINE_PANE: RenderInput<'Pane'> = {
+  ...PANE,
+  props: { ...PANE.props, placement: 'inline', bodyColumns: 156 },
+}
diff --git a/mods/diff/tests/fixtures/keeping.ts b/mods/diff/tests/fixtures/keeping.ts
new file mode 100644
index 0000000000000000000000000000000000000000..5d83ec88356b672c02c64eea8b27e21d9e60d43c
--- /dev/null
+++ b/mods/diff/tests/fixtures/keeping.ts
@@ -0,0 +1,17 @@
+/**
+ * A hook that keeps each input it is asked with, and answers, beside what
+ * it kept.
+ *
+ * @returns the hook, and each input in the order it came
+ */
+export function keeping<E>() {
+  const kept: E[] = []
+
+  function hook(_engine: unknown, e: E) {
+    kept.push(e)
+
+    return { value: undefined }
+  }
+
+  return { hook, kept }
+}
diff --git a/mods/diff/tests/fixtures/main-screen-hint.ts b/mods/diff/tests/fixtures/main-screen-hint.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fab2a36fd653fec2d5d78edb736ecb02c47dc9cd
--- /dev/null
+++ b/mods/diff/tests/fixtures/main-screen-hint.ts
@@ -0,0 +1,12 @@
+import type { RenderInput } from 'claude-code'
+
+import { HINT } from './hint.js'
+
+/**
+ * The prompt's hint on the same wide terminal on its main screen
+ * (`CLAUDE_CODE_NO_FLICKER=0`), where nothing docks and a pane opens inline.
+ */
+export const MAIN_SCREEN_HINT: RenderInput<'PromptHint'> = {
+  ...HINT,
+  viewport: { columns: 160, rows: 40, isFullscreen: false },
+}
diff --git a/mods/diff/tests/fixtures/many-file-count.ts b/mods/diff/tests/fixtures/many-file-count.ts
new file mode 100644
index 0000000000000000000000000000000000000000..25abf20b1af5ee3e876c232f6614064a3134e3f5
--- /dev/null
+++ b/mods/diff/tests/fixtures/many-file-count.ts
@@ -0,0 +1,5 @@
+/**
+ * How many files MANY_FILES changes: two more than the docked list shows at
+ * once.
+ */
+export const MANY_FILE_COUNT = 10
diff --git a/mods/diff/tests/fixtures/many-files.ts b/mods/diff/tests/fixtures/many-files.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1096139704ff86e58d2070bb496569db277d73bb
--- /dev/null
+++ b/mods/diff/tests/fixtures/many-files.ts
@@ -0,0 +1,19 @@
+import { answersOf } from './answers-of.js'
+import { MANY_FILE_COUNT } from './many-file-count.js'
+
+/**
+ * Git's output in /work where MANY_FILE_COUNT files changed a line each:
+ * more rows than the docked list shows at once.
+ */
+export const MANY_FILES = answersOf(
+  Array.from(
+    { length: MANY_FILE_COUNT },
+    (_, at) => `1\t1\tfile${at}.ts\0`,
+  ).join(''),
+  Object.fromEntries(
+    Array.from({ length: MANY_FILE_COUNT }, (_, at) => [
+      `file${at}.ts`,
+      `@@ -1 +1 @@\n-const v = ${at}\n+const v = ${at + 1}\n`,
+    ]),
+  ),
+)
diff --git a/mods/diff/tests/fixtures/moved-in.ts b/mods/diff/tests/fixtures/moved-in.ts
new file mode 100644
index 0000000000000000000000000000000000000000..43a239ca3f8af5d7e3aeb46a8aa0501f5987ff09
--- /dev/null
+++ b/mods/diff/tests/fixtures/moved-in.ts
@@ -0,0 +1,14 @@
+import { answersOf } from './answers-of.js'
+
+/**
+ * Git's output in /work where two old files differ from HEAD: one dirty
+ * before the session began, one moved into place since (not dirty then).
+ */
+export const MOVED_IN = answersOf(
+  '1\t1\told.ts\0' + '1\t1\tmoved.ts\0',
+  {
+    'old.ts': '@@ -1 +1 @@\n-a\n+b\n',
+    'moved.ts': '@@ -1 +1 @@\n-c\n+d\n',
+  },
+  ['old.ts'],
+)
diff --git a/mods/diff/tests/fixtures/narrow-inline-pane.ts b/mods/diff/tests/fixtures/narrow-inline-pane.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f17e77cb5d87fe017ed0073e4849ef68e40e156a
--- /dev/null
+++ b/mods/diff/tests/fixtures/narrow-inline-pane.ts
@@ -0,0 +1,13 @@
+import type { RenderInput } from 'claude-code'
+
+import { INLINE_PANE } from './inline-pane.js'
+
+/**
+ * The diff pane seated inline on a 100-column terminal: too narrow for the
+ * built-in panel, which shows only its wider-terminal line there.
+ */
+export const NARROW_INLINE_PANE: RenderInput<'Pane'> = {
+  ...INLINE_PANE,
+  viewport: { columns: 100, rows: 30 },
+  props: { ...INLINE_PANE.props, bodyColumns: 96 },
+}
diff --git a/mods/diff/tests/fixtures/no-stats.ts b/mods/diff/tests/fixtures/no-stats.ts
new file mode 100644
index 0000000000000000000000000000000000000000..95c0940d39ba278a463dd78c0558f0085038ebfe
--- /dev/null
+++ b/mods/diff/tests/fixtures/no-stats.ts
@@ -0,0 +1,10 @@
+import type Git from '../../hooks/git'
+
+/**
+ * The totals of a diff with nothing in it.
+ */
+export const NO_STATS: Git.DiffStats = {
+  filesCount: 0,
+  linesAdded: 0,
+  linesRemoved: 0,
+}
diff --git a/mods/diff/tests/fixtures/not-a-repository.ts b/mods/diff/tests/fixtures/not-a-repository.ts
new file mode 100644
index 0000000000000000000000000000000000000000..67ae31a8375ea321ec2b28af9ad371e4362c05c4
--- /dev/null
+++ b/mods/diff/tests/fixtures/not-a-repository.ts
@@ -0,0 +1,10 @@
+import type { ProcessRunResult } from 'claude-code'
+
+/**
+ * How git fails in a directory outside any repository.
+ */
+export const NOT_A_REPOSITORY: ProcessRunResult = {
+  exitCode: 128,
+  stdout: '',
+  stderr: 'fatal: not a git repository',
+}
diff --git a/mods/diff/tests/fixtures/old-files.ts b/mods/diff/tests/fixtures/old-files.ts
new file mode 100644
index 0000000000000000000000000000000000000000..4d71a93772dee3644fc1ab7df65368cf5bdbb2c3
--- /dev/null
+++ b/mods/diff/tests/fixtures/old-files.ts
@@ -0,0 +1,18 @@
+import type { On } from 'claude-code'
+
+/**
+ * Answers `$.fs` over /work holding MOVED_IN's two files, both last
+ * written at time 0, before any session began.
+ *
+ * @param on the test's `on`
+ */
+export function oldFiles(on: On) {
+  on('fs.list', () => ({
+    value: [
+      { name: 'old.ts', kind: 'file', size: 2 },
+      { name: 'moved.ts', kind: 'file', size: 2 },
+    ],
+  }))
+
+  on('fs.stat', () => ({ value: { kind: 'file', size: 2, mtimeMs: 0 } }))
+}
diff --git a/mods/diff/tests/fixtures/pane.ts b/mods/diff/tests/fixtures/pane.ts
new file mode 100644
index 0000000000000000000000000000000000000000..914e3071b2183903c9c25aefc5e91bb46a4792ee
--- /dev/null
+++ b/mods/diff/tests/fixtures/pane.ts
@@ -0,0 +1,20 @@
+import type { RenderInput } from 'claude-code'
+
+/**
+ * The diff pane docked on a 160-column terminal, unfocused, 80 columns of
+ * body and 30 rows in view, the main conversation showing.
+ */
+export const PANE: RenderInput<'Pane'> = {
+  component: 'Pane',
+  surface: 'terminal',
+  requestId: 'diff',
+  viewport: { columns: 160, rows: 40 },
+  props: {
+    title: 'Diff',
+    isFocused: false,
+    bodyColumns: 80,
+    placement: 'dock',
+    scroll: { offset: 0, bodyRows: 30 },
+    view: {},
+  },
+}
diff --git a/mods/diff/tests/fixtures/poll-word.ts b/mods/diff/tests/fixtures/poll-word.ts
new file mode 100644
index 0000000000000000000000000000000000000000..5e725c10c3843da5265cf030dad14b71912d2998
--- /dev/null
+++ b/mods/diff/tests/fixtures/poll-word.ts
@@ -0,0 +1,7 @@
+/**
+ * The open pane's HEAD poll as gitWordOf names it where no file answers.
+ *
+ * On disk the poll stats `.git/HEAD` and spawns nothing, so a test that
+ * lists the session's spawns as the built-in's lets this word by.
+ */
+export const POLL_WORD = 'rev-parse HEAD'
diff --git a/mods/diff/tests/fixtures/renamed.ts b/mods/diff/tests/fixtures/renamed.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c16fa0523169a1ca9bb1ccc73de029d7a368a682
--- /dev/null
+++ b/mods/diff/tests/fixtures/renamed.ts
@@ -0,0 +1,10 @@
+import { answersOf } from './answers-of.js'
+
+/**
+ * Git's output in /work where one file was renamed inside its directory,
+ * its contents untouched.
+ */
+export const RENAMED = answersOf(
+  '0\t0\t\0docs/notes.txt\0docs/renamed-notes.txt\0',
+  {},
+)
diff --git a/mods/diff/tests/fixtures/repository.ts b/mods/diff/tests/fixtures/repository.ts
new file mode 100644
index 0000000000000000000000000000000000000000..bde9f86d756c0ad7c0de486c9e315644f1f117c8
--- /dev/null
+++ b/mods/diff/tests/fixtures/repository.ts
@@ -0,0 +1,11 @@
+/**
+ * Git's output in /work, where one file changed, for each invocation whose
+ * command line holds the key.
+ */
+export const REPOSITORY: Readonly<Record<string, string>> = {
+  'rev-parse --path-format=absolute': '/work\n/work/.git\n/work/.git\n',
+  'HEAD --shortstat': ' 1 file changed, 1 insertion(+)',
+  'HEAD --numstat': '1\t0\tapp.ts\0',
+  'ls-files': '',
+  '-- app.ts': '@@ -1 +1 @@\n-const a = 1\n+const a = 2\n',
+}
diff --git a/mods/diff/tests/fixtures/ring-onto.ts b/mods/diff/tests/fixtures/ring-onto.ts
new file mode 100644
index 0000000000000000000000000000000000000000..3871a3c47dfc8b0d1e1ba0b08f7c65aae1c0b9af
--- /dev/null
+++ b/mods/diff/tests/fixtures/ring-onto.ts
@@ -0,0 +1,18 @@
+import type { UiFocusInput } from 'claude-code'
+
+import { PANE } from './pane.js'
+
+/**
+ * The person walking the pane's focus ring onto one of the plugin's
+ * elements, as the engine raises it before the ring moves.
+ *
+ * @param key the element's key
+ * @returns the `ui.focus` input
+ */
+export const ringOnto = (key: string): UiFocusInput => ({
+  component: 'Pane',
+  requestId: PANE.requestId,
+  plugin: 'diff',
+  element: key,
+  origin: { kind: 'person' },
+})
diff --git a/mods/diff/tests/fixtures/session.ts b/mods/diff/tests/fixtures/session.ts
new file mode 100644
index 0000000000000000000000000000000000000000..c387d8e755d6a8020364bae829baa466d2732f11
--- /dev/null
+++ b/mods/diff/tests/fixtures/session.ts
@@ -0,0 +1,10 @@
+import type { SessionStartInput } from 'claude-code'
+
+/**
+ * An interactive terminal session in /work.
+ */
+export const SESSION: SessionStartInput = {
+  surface: 'terminal',
+  isInteractive: true,
+  cwd: '/work',
+}
diff --git a/mods/diff/tests/fixtures/settle-ms.ts b/mods/diff/tests/fixtures/settle-ms.ts
new file mode 100644
index 0000000000000000000000000000000000000000..820789ad417757298e64cb15bb07e543ba27e2ed
--- /dev/null
+++ b/mods/diff/tests/fixtures/settle-ms.ts
@@ -0,0 +1,5 @@
+/**
+ * Longer than every wait the pane schedules before it fetches and draws
+ * (its refresh debounce and redraw), short of its HEAD poll.
+ */
+export const SETTLE_MS = 1000
diff --git a/mods/diff/tests/fixtures/starts-session.ts b/mods/diff/tests/fixtures/starts-session.ts
new file mode 100644
index 0000000000000000000000000000000000000000..9ec206a21b402ec364ba7a9462319548d7fb6f9b
--- /dev/null
+++ b/mods/diff/tests/fixtures/starts-session.ts
@@ -0,0 +1,18 @@
+import type { On } from 'claude-code'
+import { mock } from 'claude-code/testing'
+import type { MockClock } from 'claude-code/testing'
+
+/**
+ * Answers what every /diff session asks first: its start, with its own
+ * directory, the time, and each command it registers, under its name.
+ *
+ * @param on the test's `on`
+ * @param now where the clock starts, in milliseconds (0 when not given)
+ * @returns the clock the session reads, at `now` until the test moves it
+ */
+export function startsSession(on: On, now = 0): MockClock {
+  on('session.start', ($, e) => ({ cwd: e.cwd }))
+  on('command.register', ($, e) => ({ value: { command: e.name } }))
+
+  return mock.clock(on, { now })
+}
diff --git a/mods/diff/tests/fixtures/todo-write-of.ts b/mods/diff/tests/fixtures/todo-write-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e29180355c1b86aba58b795b44dda781ce6c278c
--- /dev/null
+++ b/mods/diff/tests/fixtures/todo-write-of.ts
@@ -0,0 +1,20 @@
+import type { SessionMessage } from 'claude-code'
+
+/**
+ * An assistant message holding one TodoWrite whose items have the given
+ * statuses.
+ *
+ * @param statuses each item's status
+ * @returns the message
+ */
+export const todoWriteOf = (statuses: readonly string[]): SessionMessage => ({
+  role: 'assistant',
+  text: '',
+  toolUses: [
+    {
+      tool_use_id: 'w',
+      tool: 'TodoWrite',
+      input: { todos: statuses.map(status => ({ content: 'x', status })) },
+    },
+  ],
+})
diff --git a/mods/diff/tests/fixtures/two-files.ts b/mods/diff/tests/fixtures/two-files.ts
new file mode 100644
index 0000000000000000000000000000000000000000..a6c1801be0f38d3223eb554ebbb29ba766b2a191
--- /dev/null
+++ b/mods/diff/tests/fixtures/two-files.ts
@@ -0,0 +1,9 @@
+import { answersOf } from './answers-of.js'
+
+/**
+ * Git's output in /work where two files changed, a hunk each.
+ */
+export const TWO_FILES = answersOf('1\t1\tapp.ts\0' + '2\t0\tlib.ts\0', {
+  'app.ts': '@@ -1 +1 @@\n-const a = 1\n+const a = 2\n',
+  'lib.ts': '@@ -3,0 +4,2 @@\n+export const b = 1\n+export const c = 2\n',
+})
diff --git a/mods/diff/tests/fixtures/typed-prompt-of.ts b/mods/diff/tests/fixtures/typed-prompt-of.ts
new file mode 100644
index 0000000000000000000000000000000000000000..f7523ff7fbff67ba96462f5eafb9370d09ab539e
--- /dev/null
+++ b/mods/diff/tests/fixtures/typed-prompt-of.ts
@@ -0,0 +1,19 @@
+import type { PromptSubmitInput } from 'claude-code'
+
+/**
+ * A prompt as the person submits it from the composer with a plain Enter,
+ * carrying the context given, if any.
+ *
+ * @param text what the person typed
+ * @param context what already rides beside the prompt
+ * @returns the `prompt.submit` input
+ */
+export const typedPromptOf = (
+  text: string,
+  context?: readonly string[],
+): PromptSubmitInput => ({
+  text,
+  ...(context && { context }),
+  wait: false,
+  origin: { kind: 'composer' },
+})
diff --git a/mods/diff/tests/fixtures/unsaid-hint.ts b/mods/diff/tests/fixtures/unsaid-hint.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d70bcfb41bd906f3c467994ad590c551af4fc965
--- /dev/null
+++ b/mods/diff/tests/fixtures/unsaid-hint.ts
@@ -0,0 +1,13 @@
+import type { RenderInput } from 'claude-code'
+
+import { HINT } from './hint.js'
+
+/**
+ * The prompt's hint from a surface that measured its size but did not say
+ * whether it docks a pane: an engine that predates the word, or a remote
+ * surface that has not reported it.
+ */
+export const UNSAID_HINT: RenderInput<'PromptHint'> = {
+  ...HINT,
+  viewport: { columns: 160, rows: 40 },
+}
diff --git a/mods/diff/tests/fixtures/vs-main.ts b/mods/diff/tests/fixtures/vs-main.ts
new file mode 100644
index 0000000000000000000000000000000000000000..fccb7c1f0afc942c40f87a4547d1a115a91a0b8d
--- /dev/null
+++ b/mods/diff/tests/fixtures/vs-main.ts
@@ -0,0 +1,11 @@
+import type Git from '../../hooks/git'
+
+/**
+ * A branch comparison that resolved: main as the base, at a placeholder
+ * merge-base ref.
+ */
+export const VS_MAIN: Git.DiffSource = {
+  kind: 'branch',
+  baseBranch: 'main',
+  baseRef: 'x',
+}
diff --git a/mods/diff/tests/fixtures/wheel-over-list.ts b/mods/diff/tests/fixtures/wheel-over-list.ts
new file mode 100644
index 0000000000000000000000000000000000000000..80ab7b58e6acd422d47091f96a44d3848f9e3ef0
--- /dev/null
+++ b/mods/diff/tests/fixtures/wheel-over-list.ts
@@ -0,0 +1,12 @@
+import type { UiScrollInput } from 'claude-code'
+
+import { WHEEL_TICK } from './wheel-tick.js'
+
+/**
+ * The same wheel tick with the pointer over the docked list's third row
+ * (top pad, header, the list's margin, then the rows).
+ */
+export const WHEEL_OVER_LIST: UiScrollInput = {
+  ...WHEEL_TICK,
+  pointer: { column: 10, row: 5 },
+}
diff --git a/mods/diff/tests/fixtures/wheel-tick.ts b/mods/diff/tests/fixtures/wheel-tick.ts
new file mode 100644
index 0000000000000000000000000000000000000000..e54d29e0a549b4a99a98428a128061d22ce8cdec
--- /dev/null
+++ b/mods/diff/tests/fixtures/wheel-tick.ts
@@ -0,0 +1,17 @@
+import type { UiScrollInput } from 'claude-code'
+
+import { PANE } from './pane.js'
+
+/**
+ * One tick of the person's wheel toward the foot over the docked pane, as
+ * the engine raises it before anything moves.
+ */
+export const WHEEL_TICK: UiScrollInput = {
+  component: 'Pane',
+  requestId: PANE.requestId,
+  offset: 1,
+  by: 1,
+  bodyRows: PANE.props.scroll.bodyRows,
+  contentRows: PANE.props.scroll.bodyRows + 1,
+  origin: { kind: 'person' },
+}
diff --git a/mods/diff/tests/pane-state/empty-state-of.test.ts b/mods/diff/tests/pane-state/empty-state-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..85d76c0e4ecd443b77b62959d231e508e5f11a77
--- /dev/null
+++ b/mods/diff/tests/pane-state/empty-state-of.test.ts
@@ -0,0 +1,73 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Git from '../../hooks/git'
+import PaneState from '../../hooks/pane-state'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('empty-state-of', () => {
+  const dataIn = (
+    mode: Git.BaseMode,
+    overrides: Partial<Git.DiffData> = {},
+  ): Git.DiffData => ({
+    repository: Fixtures.CHECKOUT,
+    mode,
+    stats: Fixtures.NO_STATS,
+    files: [],
+    source: { kind: 'working-tree', base: 'HEAD' },
+    isUnborn: false,
+    baseRef: 'HEAD',
+    stalePaths: [],
+    isUntrackedWithheld: false,
+    ...overrides,
+  })
+
+  const emptyOf = (data: Git.DiffData | null, filesCount: number) =>
+    PaneState.emptyStateOf({ data, words: Git.GIT_WORDS }, filesCount)
+
+  test("the built-in's headlines, keyed on the fetched mode", () => {
+    expect(emptyOf(null, 0)?.headline).toBe('Diff unavailable')
+
+    expect(emptyOf(dataIn('session'), 0)).toEqual({
+      headline: 'No changes this session',
+      hint: null,
+    })
+
+    expect(emptyOf(dataIn('uncommitted'), 0)?.headline).toBe(
+      'No uncommitted changes',
+    )
+
+    expect(
+      emptyOf(dataIn('branch', { source: Fixtures.VS_MAIN }), 0)?.headline,
+    ).toBe('No changes vs main')
+
+    expect(emptyOf(dataIn('branch'), 0)).toEqual({
+      headline: 'No changes vs HEAD',
+      hint: 'No base branch to compare against — showing changes vs HEAD',
+    })
+
+    expect(emptyOf(dataIn('session', { isUnborn: true }), 0)).toEqual({
+      headline: 'No commits yet',
+      hint: "Nothing to diff against until the repo's first commit",
+    })
+  })
+
+  test('with files to count there is no empty state', () => {
+    expect(emptyOf(dataIn('session'), 2)).toBeNull()
+  })
+
+  test('no tracked rows, untracked withheld: claims no more', () => {
+    const withheld = { isUntrackedWithheld: true }
+
+    expect(emptyOf(dataIn('session', withheld), 0)?.headline).toBe(
+      'No tracked changes',
+    )
+
+    expect(
+      emptyOf(dataIn('session', { ...withheld, isUnborn: true }), 0)?.headline,
+    ).toBe('No tracked changes')
+
+    expect(emptyOf(dataIn('uncommitted', withheld), 1)).toBeNull()
+  })
+})
diff --git a/mods/diff/tests/pane-state/header-totals-of.test.ts b/mods/diff/tests/pane-state/header-totals-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..615f66406e673fa838c637c9c8f117035d3660f3
--- /dev/null
+++ b/mods/diff/tests/pane-state/header-totals-of.test.ts
@@ -0,0 +1,74 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import type Git from '../../hooks/git'
+import PaneState from '../../hooks/pane-state'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('header-totals-of', () => {
+  const dataOf = (
+    files: readonly Git.FileStat[],
+    filesCount: number,
+  ): Git.DiffData => ({
+    repository: Fixtures.CHECKOUT,
+    mode: 'session',
+    stats: {
+      filesCount,
+      linesAdded: files.reduce((sum, each) => sum + each.added, 0),
+      linesRemoved: files.reduce((sum, each) => sum + each.removed, 0),
+    },
+    files,
+    source: { kind: 'working-tree', base: 'HEAD' },
+    isUnborn: false,
+    baseRef: 'HEAD',
+    stalePaths: [],
+    isUntrackedWithheld: false,
+  })
+
+  test('pre-session rows leave the header; the rest is not shown', () => {
+    const data = dataOf(
+      [Fixtures.SESSION_ROW, Fixtures.PRE_SESSION_ROW],
+      Fixtures.REPORTED_COUNT,
+    )
+
+    expect(
+      PaneState.headerTotalsOf(
+        data,
+        PaneState.partitionOf(data.files, 'hidden'),
+      ),
+    ).toEqual({ filesCount: 3, linesAdded: 3, linesRemoved: 1, notShown: 2 })
+  })
+
+  test('every stored row pre-session: files past the cap still count', () => {
+    const stored = Array.from({ length: Fixtures.STORED_COUNT }, (_, at) => ({
+      ...Fixtures.PRE_SESSION_ROW,
+      path: `old${at}.ts`,
+    }))
+
+    const pastCap = Fixtures.PAST_CAP_COUNT - Fixtures.STORED_COUNT
+
+    expect(
+      PaneState.headerTotalsOf(
+        dataOf(stored, Fixtures.PAST_CAP_COUNT),
+        PaneState.partitionOf(stored, 'hidden'),
+      ),
+    ).toEqual({
+      filesCount: pastCap,
+      linesAdded: 0,
+      linesRemoved: 0,
+      notShown: pastCap,
+    })
+  })
+
+  test('tests count as noise and leave the shown rows', () => {
+    const partition = PaneState.partitionOf(
+      [Fixtures.SESSION_ROW, Fixtures.TEST_ROW],
+      'hidden',
+    )
+
+    expect(partition.shown.map(each => each.path)).toEqual(['src/a.ts'])
+    expect(partition.noiseCount).toBe(1)
+    expect(PaneState.partitionOf(partition.shown, 'shown').noiseCount).toBe(0)
+  })
+})
diff --git a/mods/diff/tests/pane-state/mode-label-of.test.ts b/mods/diff/tests/pane-state/mode-label-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..52109ef73b8d9d6dfac906318e124ba13cb1f5f9
--- /dev/null
+++ b/mods/diff/tests/pane-state/mode-label-of.test.ts
@@ -0,0 +1,93 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Git from '../../hooks/git'
+import PaneState from '../../hooks/pane-state'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('mode-label-of', () => {
+  const dataIn = (
+    mode: Git.BaseMode,
+    source: Git.DiffSource,
+  ): Git.DiffData => ({
+    repository: Fixtures.CHECKOUT,
+    mode,
+    stats: Fixtures.ONE_FILE,
+    files: [],
+    source,
+    isUnborn: false,
+    baseRef: 'HEAD',
+    stalePaths: [],
+    isUntrackedWithheld: false,
+  })
+
+  const labelOf = (
+    mode: Git.BaseMode,
+    requestedMode: Git.BaseMode,
+    data: Git.DiffData | null,
+  ) =>
+    PaneState.modeLabelOf(mode, { requestedMode, data, words: Git.GIT_WORDS })
+
+  test('each settled mode is named as the built-in names it', () => {
+    const session = dataIn('session', Fixtures.WORKING_TREE)
+
+    expect(labelOf('session', 'session', session)).toBe('this session')
+
+    expect(labelOf('uncommitted', 'session', session)).toBe(
+      'uncommitted (vs HEAD)',
+    )
+
+    expect(labelOf('branch', 'session', session)).toBe('branch diff')
+  })
+
+  test('settled branch mode names its base, or what stands in', () => {
+    expect(
+      labelOf('branch', 'branch', dataIn('branch', Fixtures.VS_MAIN)),
+    ).toBe('branch vs main')
+
+    expect(
+      labelOf('branch', 'branch', dataIn('branch', Fixtures.WORKING_TREE)),
+    ).toBe('vs HEAD (no base branch)')
+  })
+
+  test('a working tree that names its own base shows that name', () => {
+    expect(
+      labelOf(
+        'uncommitted',
+        'uncommitted',
+        dataIn('uncommitted', Fixtures.AT_SHA),
+      ),
+    ).toBe('uncommitted (vs 8634408015c1)')
+  })
+
+  test('the requested mode pends with an ellipsis until its data lands', () => {
+    const session = dataIn('session', Fixtures.WORKING_TREE)
+
+    expect(labelOf('branch', 'branch', session)).toBe('branch diff…')
+    expect(labelOf('session', 'branch', session)).toBe('this session')
+
+    expect(
+      labelOf('session', 'session', dataIn('branch', Fixtures.VS_MAIN)),
+    ).toBe('this session…')
+
+    expect(labelOf('branch', 'branch', null)).toBe('branch diff')
+  })
+
+  test('an unborn HEAD with rows says what the rows are', () => {
+    const unborn = {
+      ...dataIn('session', Fixtures.WORKING_TREE),
+      isUnborn: true,
+    }
+
+    expect(PaneState.unbornNoteOf({ data: unborn }, 2)).toBe(
+      'no commits yet — showing staged and new files',
+    )
+
+    expect(PaneState.unbornNoteOf({ data: unborn }, 0)).toBeNull()
+
+    expect(
+      PaneState.unbornNoteOf({ data: dataIn('session', Fixtures.VS_MAIN) }, 2),
+    ).toBeNull()
+  })
+})
diff --git a/mods/diff/tests/pane-toggle/pane-toggle-of.test.ts b/mods/diff/tests/pane-toggle/pane-toggle-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..dfb3cb0e1c4e72b17d58abd4ea1c1c7c4d0a50aa
--- /dev/null
+++ b/mods/diff/tests/pane-toggle/pane-toggle-of.test.ts
@@ -0,0 +1,37 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Limits from '../../hooks/limits'
+import PaneToggle from '../../hooks/pane-toggle'
+
+tier('builtin')
+
+describe('pane-toggle-of', () => {
+  test('an open pane closes, however narrow', () => {
+    expect(
+      PaneToggle.paneToggleOf({
+        isOpen: true,
+        columns: Limits.OPEN_MIN_COLUMNS - 1,
+      }),
+    ).toBe('close')
+  })
+
+  test('below the panel width it is too narrow, from it it opens', () => {
+    expect(
+      PaneToggle.paneToggleOf({
+        isOpen: false,
+        columns: Limits.OPEN_MIN_COLUMNS - 1,
+      }),
+    ).toBe('too-narrow')
+
+    expect(
+      PaneToggle.paneToggleOf({
+        isOpen: false,
+        columns: Limits.OPEN_MIN_COLUMNS,
+      }),
+    ).toBe('open')
+
+    expect(PaneToggle.paneToggleOf({ isOpen: false, columns: null })).toBe(
+      'open',
+    )
+  })
+})
diff --git a/mods/diff/tests/todos/todo-progress-of.test.ts b/mods/diff/tests/todos/todo-progress-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..d5f8040c71d82e2b1da51e376b73710225edf556
--- /dev/null
+++ b/mods/diff/tests/todos/todo-progress-of.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Todos from '../../hooks/todos'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('todo-progress-of', () => {
+  test('the latest TodoWrite decides: completed over total', () => {
+    expect(
+      Todos.todoProgressOf([
+        Fixtures.todoWriteOf(['pending', 'pending']),
+        Fixtures.todoWriteOf(['completed', 'in_progress', 'pending']),
+      ]),
+    ).toEqual({ done: 1, total: 3 })
+  })
+
+  test('no TodoWrite yet reads as an empty list', () => {
+    expect(
+      Todos.todoProgressOf([{ role: 'user', text: 'hi', toolUses: [] }]),
+    ).toEqual({
+      done: 0,
+      total: 0,
+    })
+  })
+})
diff --git a/mods/diff/tests/turns/turn-diffs-of.test.ts b/mods/diff/tests/turns/turn-diffs-of.test.ts
new file mode 100644
index 0000000000000000000000000000000000000000..0fcffddf80dcaf65454af86113f75b4b26cc23fb
--- /dev/null
+++ b/mods/diff/tests/turns/turn-diffs-of.test.ts
@@ -0,0 +1,94 @@
+import { describe, expect, test, tier } from 'claude-code/testing'
+
+import Limits from '../../hooks/limits'
+import Turns from '../../hooks/turns'
+import Fixtures from '../fixtures'
+
+tier('builtin')
+
+describe('turn-diffs-of', () => {
+  test('turns with edits, newest first, numbered by prompt', () => {
+    const turns = Turns.turnDiffsOf([
+      Fixtures.promptOf('first'),
+      Fixtures.editsOf(Fixtures.editPatchOf(['-a', '+b'])),
+      Fixtures.toolResults(),
+      Fixtures.promptOf('second, nothing edited'),
+      Fixtures.promptOf('third turn with a long prompt that gets cut'),
+      Fixtures.editsOf(Fixtures.editPatchOf(['+c']), {
+        filePath: '/r/new.ts',
+        type: 'create',
+        content: 'x\ny',
+      }),
+      Fixtures.toolResults(),
+    ])
+
+    expect(turns.map(turn => turn.index)).toEqual([3, 1])
+    expect(turns[0]?.preview).toBe('third turn with a long prompt…')
+
+    expect(
+      turns[0]?.files.map(file => [
+        file.path,
+        file.added,
+        file.removed,
+        file.isNewFile,
+      ]),
+    ).toEqual([
+      ['/r/a.ts', 1, 0, false],
+      ['/r/new.ts', 2, 0, true],
+    ])
+
+    expect(turns[0]?.files[1]?.hunks).toEqual([
+      { oldStart: 0, newStart: 1, lines: ['+x', '+y'] },
+    ])
+
+    expect(turns[1]?.files.map(file => [file.added, file.removed])).toEqual([
+      [1, 1],
+    ])
+  })
+
+  test('a file edited twice in a turn: both hunks, summed', () => {
+    const [turn] = Turns.turnDiffsOf([
+      Fixtures.promptOf('go'),
+      Fixtures.editsOf(
+        Fixtures.editPatchOf(['+a']),
+        Fixtures.editPatchOf(['-b', '+c']),
+      ),
+    ])
+
+    expect(turn?.files).toHaveLength(1)
+    expect(turn?.files[0]?.hunks).toHaveLength(2)
+    expect([turn?.files[0]?.added, turn?.files[0]?.removed]).toEqual([2, 1])
+  })
+
+  test('results that are not file edits are ignored', () => {
+    expect(
+      Turns.turnDiffsOf([
+        Fixtures.promptOf('go'),
+        Fixtures.editsOf({ stdout: 'x' }, 'text', null),
+      ]),
+    ).toEqual([])
+  })
+
+  test('a big created file is cut at the line cap and says so', () => {
+    const [turn] = Turns.turnDiffsOf([
+      Fixtures.promptOf('write it'),
+      Fixtures.editsOf({
+        filePath: '/r/big.ts',
+        type: 'create',
+        content: Array.from(
+          { length: Fixtures.BIG_LINES },
+          (_, at) => `l${at}`,
+        ).join('\n'),
+      }),
+    ])
+
+    const file = turn?.files[0]
+
+    expect(file?.hunks.flatMap(hunk => hunk.lines)).toHaveLength(
+      Limits.MAX_LINES_PER_FILE,
+    )
+
+    expect(file?.isTruncated).toBe(true)
+    expect(file?.added).toBe(Fixtures.BIG_LINES)
+  })
+})