| # Timeline Layout Continuity |
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| Run from `packages/app`: |
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| ```sh |
| bun run test:stability |
| ``` |
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| The suite runs a production build in one Chromium worker. Selected scenarios use deterministic 4x CPU stress after application readiness. This is a stress profile, not emulation of a specific device. |
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| ## What It Proves |
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| The continuity probe samples DOM-derived layout and visibility state across browser render opportunities. Tests declare explicit contracts such as: |
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| - Preserve a visible semantic anchor while the user is away from the bottom. |
| - Preserve end anchoring while active content grows or new content appears. |
| - Keep adjacent visible rows ordered without material overlap. |
| - Keep user-selected disclosure state through updates and virtualization. |
| - Avoid a sampled blank interval while one visible surface replaces another. |
| - Preserve logical row and control identity where local state or focus depends on it. |
| - Keep keyboard, wheel, and nested-scroll ownership consistent during remeasurement. |
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| The suite exercises real browser reducer, projection, component, virtualizer, layout, focus, and interaction code. The backend and event producer are controlled fixtures. |
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| ## What It Does Not Prove |
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| The pass/fail oracle does not inspect every compositor-presented pixel. A sample taken after `requestAnimationFrame` is a DOM/layout observation, not proof that every sampled state was displayed or that every displayed frame was sampled. |
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| The suite does not provide complete coverage for: |
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| - Compositor-only or raster-only glitches. |
| - Color, contrast, canvas, WebGL, masks, irregular clips, or arbitrary occlusion. |
| - Physical display refresh rates, native OS scaling, or a named low-end device. |
| - TCP packetization, proxy buffering, or the complete real server/provider pipeline. |
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| Playwright video, trace, screenshots, and observation JSON are diagnostic evidence. They are not pixel baselines and do not participate in normal pass/fail decisions. |
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| For optional before/violation/after screenshots, set `OPENCODE_STABILITY_CAPTURE=1`. Capture is opt-in because compositor readback can perturb timing. |
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| ## Test Layers |
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| - **Projection:** admitted rows, grouping, labels, and final visible states. |
| - **Local state:** disclosure state, identity, duplicate delivery, and virtualization restoration. |
| - **Interaction:** wheel, keyboard, nested scrolling, actionability, and focus behavior. |
| - **Layout continuity:** anchoring, adjacency, responsive reflow, and visible surface handoffs. |
| - **Reducer hardening:** validly shaped but intentionally reordered, duplicated, removed, or replaced events. |
| - **Oracle contract:** pure analyzer and browser sampler calibration tests. |
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| Production-lifecycle fixtures should model states emitted by the current producer. Impossible or reordered sequences belong in reducer-hardening tests and must not be described as normal provider behavior. |
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| ## Diagnostics |
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| Failures retain: |
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| - `video.webm` |
| - `trace.zip` |
| - failure screenshot |
| - sampled DOM/layout trace JSON |
| - event markers and summarized violations |
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| The analyzer records both unclipped layout bounds and ancestor-clipped visible intersections. Scrollbar and raw `scrollTop` changes alone do not fail continuity checks; user-visible semantic anchor movement does. |
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