src/app/main.cpp

// FCS Glass - src/app/main.cpp
//
// MILESTONE M0: a real window, a real device, and the first presented frame.
// The smallest program that can honestly be called "running": one top-level
// window, one DComp swap chain, one Present. Everything visual (glass, blur,
// text, widgets) is M2+ and arrives behind the same frame loop.
//
// RESEARCH FINDINGS - each read from the SDK or measured before it was written.
//
//  1. Visual tree order is fixed and not interchangeable (learn.microsoft.com,
//     "How to build a simple visual tree"):
//        DCompositionCreateDevice(IDXGIDevice*)      <- from the DXGI device, not QI
//        IDCompositionDevice::CreateTargetForHwnd
//        IDXGIFactory2::CreateSwapChainForComposition
//        IDCompositionDevice::CreateVisual
//        IDCompositionVisual::SetContent(swap chain)
//        IDCompositionTarget::SetRoot(visual)
//        IDCompositionDevice::Commit
//     SetContent takes an IUnknown*, so the swap chain IS the content - there is
//     no intermediate surface to allocate. Getting this order wrong yields a
//     window that stays black while every HRESULT reports S_OK, which is why
//     each step is checked separately (D-22, D-23, D-25).
//
//  2. The DPI manifest is a two-element ladder, not one element. On a host older
//     than Windows 10 1607, <dpiAwareness> is IGNORED, so <dpiAware>true/pm</dpiAware>
//     is the fallback and the process does not silently become DPI-unaware on the
//     2012 R2 floor. See src/app/app.manifest.
//
//  3. A composition swap chain MUST use DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL. Measured:
//     DISCARD returns DXGI_ERROR_INVALID_CALL (0x887A0001), as does BufferCount 0.
//     (D-25)
//
//  4. The first frame is presented BEFORE the loop. Presenting inside the loop
//     leaves a visible empty window for one refresh, and makes a later DwmFlush()
//     measure the interval since the PREVIOUS present rather than this frame -
//     which is what made the M0a probe report the 16.66 ms vsync period as if it
//     were a frame cost (X29; J6 splits pacing from GPU cost for this reason).
//
//  5. Three API details the first draft got wrong, all caught by the compiler
//     rather than by reading:
//       a. ComPtr (platform.h) has explicit operator bool and operator! but NO
//          operator==, so `ptr == nullptr` is error C2678. Use `!ptr`.
//       b. ComPtr's out-parameter accessor is GetAddressOf(), not GetAddrOf().
//       c. IDCompositionDevice::CreateTargetForHwnd takes THREE arguments
//          (dcomp.h L266): (HWND, BOOL topmost, IDCompositionTarget**). The
//          `topmost` flag is not optional - IID_PPV_ARGS supplies only the third.
//       d. OsProfile is Classic/Fluent (W4), not a Windows-version name, and
//          build is a DWORD, so %lu needs an explicit unsigned long cast.
//
// Not in this milestone, deliberately: no shader is compiled and none is drawn.
// M0 presents the swap chain and nothing more. J11's embedded bytecode path needs
// shaders/glass.hlsl compiled by fxc, which is M2; adding a placeholder draw here
// would make M2 look further along than it is.

#include <windowsx.h>   // GET_X_LPARAM / GET_Y_LPARAM for WM_DPICHANGED (E1)

#include "platform/platform.h"
#include "platform/os_info.h"
#include "platform/dpi.h"
#include "platform/window.h"
#include "platform/backdrop.h"
#include "platform/input_map.h"
#include "device/dx.h"
#include "core/input.h"

#include <cstdio>

namespace fcs {
namespace {

// BUG-26. The system's modal size/move loop OWNS THIS THREAD between
// WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE, so RunApp's frame loop cannot iterate
// while the user drags an edge. The client grows immediately (WM_NCCALCSIZE is
// now correct), but the swap chain still holds the PREVIOUS, smaller buffer -
// so the strip the window has just gained has no composition content at all and
// composites black. It fills in only once the drag ends and the loop finally
// runs, which is exactly the "black while I drag, then the teal catches up"
// behaviour that was reported.
//
// The demo in this workspace hit the same wall and solved it the same way:
// repaint synchronously from the message that reports the resize, so the
// presented frame and the window rectangle stay in lockstep. Outside a drag the
// frame loop still owns rendering; nothing here changes the idle model.
//
// This is file-scope because HostWndProc and RunApp are two different functions
// and the WndProc has only an HWND to work from. It is the same shape as the
// demo's g_swapchain/g_in_size_move, deliberately.
struct FrameCtx {
  DxDevice* dx = nullptr;
  Window* win = nullptr;
  QualityTier tier = QualityTier::Full;
  // C3/C4/E11. The one InputFrame for the process, fed by HostWndProc and frozen
  // once per frame by RunApp. It lives here for the same reason `win` and `dx`
  // do: the window procedure has only an HWND to work from.
  core::InputFrame input;
  // The size the chain was last SUCCESSFULLY sized to, shared by both paths so
  // a synchronous resize does not leave the loop believing it still has work.
  UINT last_w = 0;
  UINT last_h = 0;
  bool in_size_move = false;
  // Reentrancy guard. A D3D/DXGI call can dispatch messages, and a present
  // started from inside a present is how this kind of code deadlocks.
  bool in_flight = false;
  // BUG-26 diagnostics: emit at most ONE "sync" line per drag. TraceDrag opens
  // trace.txt on every call, so a line per WM_SIZE would slow the very drag it
  // is measuring.
  bool sync_traced = false;
  // BUG-26: the same bounding trick for the WM_WINDOWPOSCHANGING pre-stage, so
  // a drag that produced NO `pre` line proves the pre-stage never ran, rather
  // than leaving us guessing whether the hook fired.
  bool prestage_traced = false;
  // BUG-26: true between WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE while the chain
  // is deliberately OVERSIZED, so no ResizeBuffers happens per mouse-move.
  bool oversize_active = false;
};
FrameCtx g_frame;

// PresentFrame and its result enum are defined further down this file, but
// HostWndProc's WM_WINDOWPOSCHANGING pre-stage calls it, so both are declared
// here. The default argument lives on the declaration, not the definition.
enum class PresentResult { Presented, Retry, Failed };
PresentResult PresentFrame(DxDevice& dx, bool allow_wait = false);

// Defined below (they need PresentFrame); declared here because the window
// procedure above them needs to call them.
bool EnsureChainForSize(UINT w, UINT h);
bool EnsureChainCoversClient();
void SyncResizeAndPresent();
void TraceDrag(const char* tag);

// R6/C4: input is a dirty source, but NOT every input event may be one. A
// mouse-move fires continuously, so dirtying on each would hold a permanent
// 60 fps redraw on an idle window - the exact busy-wait this loop was built to
// eliminate. Discrete events (buttons, wheel, keys, characters) always dirty. A
// move dirties only while a drag is in flight; until there are widgets, a hover
// change has nothing to redraw, and the snapshot carries it for free anyway.
inline void MarkDirtyForInput(Window* win, bool dirty) {
  if (win != nullptr && dirty) win->MarkDirty();
}

// D-15/D-16: the host window procedure. Routing lives here; state decisions stay in
// Window, and the DComp lifecycle stays in RunApp, where the ordering is visible.
LRESULT CALLBACK HostWndProc(HWND hwnd, UINT msg, WPARAM w, LPARAM l) {
  // WM_NCCALCSIZE is delivered DURING CreateWindowEx, before GWLP_USERDATA is
  // set, so the null check below used to swallow it and hand it to
  // DefWindowProc. That applied the default system frame, and the client area
  // was born 22x56 px smaller than the window and never corrected: measured
  // 1280x800 window -> 1258x744 client. The uncovered margin composited black
  // because the swap chain is premultiplied. It must be handled first, from the
  // HWND alone.
  if (msg == WM_NCCALCSIZE) {
    // lParam is an NCCALCSIZE_PARAMS*, NOT a CREATESTRUCTW* - see the J7 note in
    // probe.cpp, where guessing that faulted the callback (0xC000041D).
    return Window::NccCalcSizeFor(hwnd, w, l);
  }

  Window* win = reinterpret_cast<Window*>(::GetWindowLongPtrW(hwnd, GWLP_USERDATA));
  if (win == nullptr) return ::DefWindowProcW(hwnd, msg, w, l);

  switch (msg) {
    case WM_NCCALCSIZE: {
      // lParam is an NCCALCSIZE_PARAMS*, NOT a CREATESTRUCTW* - see the J7 note in
      // probe.cpp, where guessing that faulted the callback (0xC000041D).
      return win->OnNCCalcSize(w, l);
    }

    case WM_NCHITTEST: {
      // W2. THIS CASE IS THE REASON THE WINDOW WAS IMMOBILE.
      //
      // HostWndProc had no WM_NCHITTEST at all, so every one of these went to
      // DefWindowProc, which for a plain WS_OVERLAPPEDWINDOW answers HTCLIENT
      // for the whole window. The user-visible result was a window that could
      // not be dragged, not resized, and whose caption buttons did nothing:
      // Window::NCHitTest, HitTestClient, HitToHt and the whole caption-button
      // cluster in window.cpp were implemented, unit-reachable, and NEVER CALLED.
      //
      // Because WM_NCCALCSIZE returns 0 the client area covers the frame, so the
      // caption strip and resize borders are inside the CLIENT rect and only a
      // hit test can classify them. lParam is SCREEN coordinates here; the
      // ScreenToClient conversion lives in NCHitTest and happens exactly once.
      POINT pt = {GET_X_LPARAM(l), GET_Y_LPARAM(l)};
      return win->NCHitTest(pt);
    }

    case WM_NCMOUSEMOVE:
      // W2/W3. Hover state for the caption cluster. This is a non-client message
      // precisely because HTCAPTION routes input away from the client window.
      win->OnNcMouseMove(POINT{GET_X_LPARAM(l), GET_Y_LPARAM(l)});
      return 0;

    case WM_NCMOUSELEAVE:
      win->OnNcMouseLeave();
      return 0;

    case WM_NCLBUTTONDOWN:
    case WM_NCLBUTTONUP:
    case WM_NCRBUTTONUP:
      // W2/W9. Delegated to DefWindowProc on purpose. The hit test now returns
      // HTCAPTION for the strip and HTMINBUTTON/HTMAXBUTTON/HTCLOSE for the
      // buttons, and the OS drives all of them from a real frame: HTCAPTION
      // starts the drag, and the three button codes run the standard commands.
      //
      // Calling Window::OnNcLButtonUp here instead would DOUBLE-ACT - the
      // maximize toggle would flip twice and restore itself. Window keeps those
      // handlers for the NOACTIVATE dock window (M1+), which has no OS frame to
      // drive it; this main window is not that window.
      return ::DefWindowProcW(hwnd, msg, w, l);

    case WM_GETMINMAXINFO: {
      // W6. A minimum size, so a resize cannot produce an unusable window.
      // W6/BUG-05. The Window converts its dp minimum to px at the CURRENT
      // dpi, at query time. The old code read a pre-converted pair that
      // was initialised once (640x420 physical px) and never recomputed.
      win->OnGetMinMaxInfo(reinterpret_cast<MINMAXINFO*>(l));
      return 0;
    }

    case WM_SYSCOMMAND: {
      // W1/J7. Track the zoomed state, because WM_NCCALCSIZE needs to know it
      // to apply the maximized inset.
      if ((w & 0xFFF0) == SC_MAXIMIZE) {
        win->set_maximized(true);
      } else if ((w & 0xFFF0) == SC_RESTORE || (w & 0xFFF0) == SC_MINIMIZE) {
        win->set_maximized(false);
      }
      break;   // fall through to DefWindowProc
    }

    case WM_ERASEBKGND:
      // E2: returning 1 is what removes the black flash on maximize and snap.
      // Returning 0 lets Windows paint a black canvas we then overwrite, which is
      // precisely the flicker E2 forbids.
      return 1;

    case WM_DWMCOMPOSITIONCHANGED:
      // W1: the margins call must be repeated here, or a composition restart
      // silently drops the sheet of glass.
      win->OnCompositionChanged();
      win->MarkDirty();   // BUG-01: the recomposited surface must be re-presented
      return 0;

    case WM_DPICHANGED:
      // E1/BUG-06/RV-07. The handler lives in Window, which LEDGER P1 already
      // names as the owner - and which the probe can therefore drive directly,
      // so RV-07's "the guard cannot get stuck" fix has a test.
      return win->OnDpiChanged(w, l);

    case WM_WINDOWPOSCHANGING: {
      // BUG-26, third part - the measured one. Every handler above runs AFTER
      // the window rect has already changed, so each presents into a window that
      // is already larger than its buffer. trace.txt measured that shortfall -
      // logged BEFORE the fix-up - at 4-11 px at the first synchronous present
      // of each drag, and that thin strip is the residual dark edge. Re-arming
      // on WM_SIZE or WM_PAINT cannot close it: both are dispatched after the
      // fact, one mouse-move step too late.
      //
      // WM_WINDOWPOSCHANGING is delivered BEFORE the move/resize is applied, so
      // staging the buffer here puts the frame on screen BEFORE the rect grows.
      // Scoped to in_size_move so ordinary moves, maximize and snap keep using
      // the established paths - which also keeps the staged size correct,
      // because outside a zoom transition our client equals the window, so cx/cy
      // IS the client size and the following WM_SIZE then finds the chain already
      // at the right size and does not resize a second time.
      if (g_frame.in_size_move && !g_frame.oversize_active && !g_frame.in_flight &&
          g_frame.dx != nullptr && g_frame.win != nullptr) {
        const WINDOWPOS* wp = reinterpret_cast<const WINDOWPOS*>(l);
        if (wp != nullptr && (wp->flags & SWP_NOSIZE) == 0 && wp->cx > 0 &&
            wp->cy > 0 && (wp->flags & SWP_HIDEWINDOW) == 0 &&
            !::IsIconic(g_frame.win->handle())) {
          const UINT stage_w = static_cast<UINT>(wp->cx);
          const UINT stage_h = static_cast<UINT>(wp->cy);
          if (stage_w != g_frame.last_w || stage_h != g_frame.last_h) {
            g_frame.in_flight = true;
            if (EnsureChainForSize(stage_w, stage_h)) {
              PresentFrame(*g_frame.dx, /*allow_wait=*/true);
            }
            g_frame.in_flight = false;
            // Logged AFTER the resize, so a `pre` line whose buffer already
            // equals the window is direct proof the hook fired in time.
            if (!g_frame.prestage_traced) {
              g_frame.prestage_traced = true;
              TraceDrag("pre");
            }
          }
        }
      }
      break;   // the system must still apply the move/resize
    }

    case WM_ENTERSIZEMOVE:
      // BUG-26: from here until WM_EXITSIZEMOVE the system owns the mouse AND
      // this thread, so the frame loop below cannot iterate. Every resize that
      // happens in that window of time must be presented from here instead.
      g_frame.in_size_move = true;
      g_frame.sync_traced = false;
      g_frame.prestage_traced = false;

      // BUG-26, architecture change. Resizing the chain on every mouse-move
      // calls ResizeBuffers dozens of times a second, and each call tears the
      // back buffer down and allocates a new one. The trace had already proved
      // the buffer was big enough - `sync` reported 0 px behind the window - yet
      // a dark band still showed, so the residual was never a coverage hole: it
      // was the churn itself. So the chain is sized ONCE here, generously, and
      // then left alone for the whole drag. The surplus is clipped by the window
      // rect and costs nothing. Two ResizeBuffers per drag instead of fifty.
      if (g_frame.dx != nullptr && g_frame.win != nullptr) {
        RECT wr0 = {};
        ::GetWindowRect(hwnd, &wr0);
        const UINT cur_w = static_cast<UINT>(wr0.right - wr0.left);
        const UINT cur_h = static_cast<UINT>(wr0.bottom - wr0.top);
        const int margin = DpToPx(200.0f, g_frame.win->dpi_scale());
        const UINT wide_w = cur_w + static_cast<UINT>(margin * 2);
        const UINT wide_h = cur_h + static_cast<UINT>(margin * 2);
        g_frame.oversize_active = true;
        if (EnsureChainForSize(wide_w, wide_h)) {
          PresentFrame(*g_frame.dx, /*allow_wait=*/false);
        }
        TraceDrag("wide");
      }
      return 0;

    case WM_EXITSIZEMOVE:
      // Drop the oversized buffer and land on the exact final client size: the
      // last WM_SIZE of a drag can arrive before the window settles, and the
      // loop may not have reached its resize block yet.
      g_frame.in_size_move = false;
      g_frame.oversize_active = false;
      SyncResizeAndPresent();
      TraceDrag("exit");
      return 0;

    case WM_SIZE: {
      // RV-07: the guard has to be READ HERE, before the line that clears it.
      // Read after EndDpiChange it is unconditionally false, so the guard guards
      // nothing - which is the state this flag has been in since it was added:
      // written by WM_DPICHANGED, cleared by this very message, read by nobody.
      // E1's "does not reflow twice" guarantee was therefore vacuous, and the
      // M1 dock layout that is meant to consume it had nothing to consume.
      const bool provoked_by_dpi = win->dpi_change_in_flight();
      win->EndDpiChange();
      // BUG-01: this message is usually dispatched by the system's modal
      // move/size pump, which never runs the app's idle GetMessageW branch,
      // so the dirty flag is raised HERE, on the Window, where every pump
      // can reach it. Clearing is the frame loop's job.
      win->MarkDirty();
      // BUG-26: during a drag the frame is presented from HERE, synchronously, so
      // the composited frame always covers the rectangle it is being shown in.
      // E1: a WM_SIZE provoked by WM_DPICHANGED must NOT present - that handler
      // is still mid-flight and the suggested rect is not applied yet, so
      // presenting here is precisely the double reflow E1 exists to prevent. The
      // dirty flag is already set, so the frame loop presents the settled size
      // on the very next iteration.
      if (g_frame.in_size_move && !provoked_by_dpi) SyncResizeAndPresent();
      return 0;
    }

    case WM_PAINT:
      // BUG-01: WM_PAINT may be dispatched by ANY pump (ours, the modal
      // move/size pump, a nested loop). DefWindowProc then validates the
      // update region, which is the ONLY notification a resize exposes -
      // so the dirty flag must be raised before falling through.
      win->MarkDirty();
      // BUG-26, second half. A resize INVALIDATES the window, and mid-drag that
      // WM_PAINT is dispatched by the system's modal loop, where the frame loop
      // cannot run. Raising the dirty flag alone therefore changes nothing until
      // the mouse is released, and the newly exposed region stays dark for that
      // whole time - which is exactly the residual "dark window for a split
      // second" reported after the WM_SIZE fix. Answer it HERE; DefWindowProc
      // still runs below and validates the update region.
      if (g_frame.in_size_move) SyncResizeAndPresent();
      return ::DefWindowProcW(hwnd, msg, w, l);

    // ---- input (C3, C4, E11) ------------------------------------------------
    // Everything here TRANSLATES a Win32 message into a core-layer call. No
    // decisions are made here and none of it is testable in the portable
    // harness; that is the boundary the split is for.

    case WM_MOUSEMOVE:
      // Client coordinates - lParam is already in client space for this message.
      g_frame.input.OnMouseMove(GET_X_LPARAM(l), GET_Y_LPARAM(l));
      MarkDirtyForInput(win, g_frame.input.buttons_down() != 0);   // drag only
      return 0;

    case WM_MOUSEWHEEL: {
      // Three things are wrong with reading this like a mouse-move, and all
      // three are silent:
      //   1. The delta is in the HIGH word of wParam, in 120ths of a notch.
      //    GET_X_LPARAM(l) reads the LOW word, which is not the delta.
      //   2. The position in lParam is SCREEN space, not client space.
      //   3. The delta is POSITIVE when the wheel goes up, and Windows'
      //    convention is that a positive wheel means "away from the user".
      g_frame.input.OnWheel(GET_WHEEL_DELTA_WPARAM(w));
      POINT pt = {GET_X_LPARAM(l), GET_Y_LPARAM(l)};
      ::ScreenToClient(hwnd, &pt);
      g_frame.input.OnMouseMove(pt.x, pt.y);
      MarkDirtyForInput(win, true);
      return 0;
    }

    case WM_LBUTTONDOWN:
    case WM_RBUTTONDOWN:
    case WM_MBUTTONDOWN: {
      const int b = platform::ButtonFromMessage(msg);   // P3-01: never from wParam
      if (b >= 0) {
        g_frame.input.OnMouseMove(GET_X_LPARAM(l), GET_Y_LPARAM(l));
        // E11: SetCapture is what lets a drag continue outside the client.
        // Without it WM_MOUSEMOVE stops at the window edge, the drag freezes
        // mid-gesture, and the button is still down when the pointer returns.
        ::SetCapture(hwnd);
        // Armed only when a drag starts. A TrackMouseEvent on every
        // WM_MOUSEMOVE is a syscall per mouse move, and with no button held
        // there is nothing for a leave notification to invalidate.
        TRACKMOUSEEVENT tme = {sizeof(tme), TME_LEAVE, hwnd, 0};
        ::TrackMouseEvent(&tme);
        g_frame.input.OnMouseButton(b, true);
      }
      MarkDirtyForInput(win, true);
      return 0;
    }

    case WM_XBUTTONDOWN:
    case WM_XBUTTONUP: {
      // X buttons put the button number in the HIGH word, so they cannot go
      // through ButtonFromVk at all.
      const int b = 3 + (GET_XBUTTON_WPARAM(w) == XBUTTON2 ? 1 : 0);
      g_frame.input.OnMouseMove(GET_X_LPARAM(l), GET_Y_LPARAM(l));
      if (msg == WM_XBUTTONDOWN) {
        ::SetCapture(hwnd);
        TRACKMOUSEEVENT tme = {sizeof(tme), TME_LEAVE, hwnd, 0};
        ::TrackMouseEvent(&tme);
      }
      g_frame.input.OnMouseButton(b, msg == WM_XBUTTONDOWN);
      // P3-07: mirror the left/right/middle path. Capture was taken on down and must be
      // dropped once NOTHING is held, or the pointer stays captured after the click.
      if (msg == WM_XBUTTONUP &&
          (g_frame.input.buttons_down() & ~core::InputFrame::ButtonBit(b)) == 0) {
        ::ReleaseCapture();
      }
      MarkDirtyForInput(win, true);
      return TRUE;   // documented return value for a handled XBUTTON message
    }

    case WM_LBUTTONUP:
    case WM_RBUTTONUP:
    case WM_MBUTTONUP: {
      const int b = platform::ButtonFromMessage(msg);   // P3-01: never from wParam
      if (b >= 0) {
        g_frame.input.OnMouseMove(GET_X_LPARAM(l), GET_Y_LPARAM(l));
        g_frame.input.OnMouseButton(b, false);
        // Release the capture only once NOTHING is still held. Dropping it
        // while a second button is down silently ends that button's drag.
        if ((g_frame.input.buttons_down() & ~core::InputFrame::ButtonBit(b)) == 0) {
          ::ReleaseCapture();
        }
      }
      MarkDirtyForInput(win, true);
      return 0;
    }

    case WM_MOUSELEAVE:
      // No button-up is delivered when the pointer leaves mid-press, so the
      // button would otherwise stay down until the user clicks again.
      g_frame.input.ReleaseAllButtons();
      return 0;

    case WM_CAPTURECHANGED:
      // E11: the capture can be taken by another window, a UAC prompt, or a
      // session switch, and Windows delivers NO button-up when it goes. A drag
      // left running here keeps reporting a pointer position nobody can move,
      // and the next click would then look like a second concurrent drag.
      g_frame.input.OnCaptureChanged();
      return 0;

    case WM_KEYDOWN:
    case WM_SYSKEYDOWN: {
      const core::Key k = platform::KeyFromVk(static_cast<int>(w));
      if (k != core::Key::None) {
        g_frame.input.OnKey(k, true);
        MarkDirtyForInput(win, true);
      }
      break;   // DefWindowProc still owns Alt/System-menu handling
    }

    case WM_KEYUP:
    case WM_SYSKEYUP: {
      const core::Key k = platform::KeyFromVk(static_cast<int>(w));
      if (k != core::Key::None) {
        g_frame.input.OnKey(k, false);
        MarkDirtyForInput(win, true);
      }
      break;
    }

    case WM_CHAR:
      // E11: wParam is ONE UTF-16 CODE UNIT, not a character. Every non-BMP
      // character arrives as two of these, and a text field that emits them
      // separately fills with U+FFFD. WM_SYSCHAR is deliberately NOT handled -
      // Alt+key is a menu accelerator, not text.
      g_frame.input.OnCharUnit(static_cast<unsigned>(w));
      MarkDirtyForInput(win, true);
      return 0;

    case WM_KILLFOCUS:
      // E11: Windows does NOT deliver the matching WM_KEYUP when focus is lost,
      // so every key held during an Alt+Tab would stay down for the rest of the
      // session. Same for a button held while a menu took the click.
      g_frame.input.ReleaseAllKeys();
      g_frame.input.ReleaseAllButtons();
      return 0;

    case WM_CLOSE:
      ::DestroyWindow(hwnd);
      return 0;

    case WM_DESTROY:
      ::PostQuitMessage(0);
      return 0;
  }
  return ::DefWindowProcW(hwnd, msg, w, l);
}

// M0. Build the composition content for this window.
//
// OWNERSHIP: DxDevice::CreateSwapChain OWNS the composition tree (target, root
// visual, SetContent, Commit). This function used to build a second one over the
// same swap chain, which gave one HWND two DComp targets and made the winner
// depend on an inconsistent `topmost` flag (TRUE in dx.cpp, FALSE here). It now
// only asks the device to (re-)attach, and delegates the tree to its owner.
bool BuildCompositionTree(DxDevice& dx, Window& win, QualityTier tier) {
  if (!dx.composition()) return false;

  RECT cr = {};
  if (!::GetClientRect(win.handle(), &cr)) return false;
  const UINT w = static_cast<UINT>(cr.right - cr.left);
  const UINT h = static_cast<UINT>(cr.bottom - cr.top);
  if (w == 0 || h == 0) return false;   // E2: never present a zero-sized chain

  // A chain that already exists at this size only needs its content re-linked
  // (the resize case). BUG-14: if that re-link fails the chain is unusable
  // (device removed, failed resize) - fall through and rebuild the whole
  // chain + tree instead of reporting a failure the caller cannot recover.
  if (dx.swap_chain() != nullptr && dx.AttachContent()) return true;
  return dx.CreateSwapChain(win.handle(), w, h, tier);
}

// M0. One frame: present the current back buffer.
//
// BUG-02. The result distinguishes "frame is on the screen" from "frame was
// dropped, try again". The old signature returned bool and mapped
// DXGI_ERROR_WAS_STILL_DRAWING to true, so the caller cleared the dirty
// flag for a present that never happened - the first frame could be lost
// and the window would sit on its DComp-default black until a later event.
// The enum is declared with the forward declarations at the top of this file,
// because HostWndProc's WM_WINDOWPOSCHANGING pre-stage calls PresentFrame.
//
// J5/G1: the chain is B8G8R8A8 with premultiplied alpha (X28), so anything drawn
// into it must be given PREMULTIPLIED. J6 note: this Present is what a later
// DwmFlush() would be measured against, so it must happen before the wait - that
// ordering is exactly what X29 got wrong in the M0a probe.
PresentResult PresentFrame(DxDevice& dx, bool allow_wait) {
  if (dx.swap_chain() == nullptr) return PresentResult::Failed;

  // M0 presents a CLEARED frame. The first version of this function called
  // Present() and nothing else, so the back buffer was whatever the driver left
  // in it - which on this host is black. A swap chain that is presented without
  // ever being written is not "a presented frame", it is an undefined one, and
  // there is no way to tell a working renderer from a dead one by looking.
  //
  // The clear is deliberately a distinctive mid slate-teal rather than black,
  // for two reasons: a black clear is indistinguishable from the failure above,
  // and a non-black fill is positive evidence that THESE pixels came from THIS
  // Present. That is what makes a screenshot able to prove the frame arrived.
  HRESULT hr = S_OK;
  ID3D11RenderTargetView* rtv = dx.AcquireRtv(&hr);
  if (rtv == nullptr || FAILED(hr)) {
    FCS_CHECK_HR(hr);
    return PresentResult::Failed;
  }
  ID3D11DeviceContext* ctx = dx.context();
  ctx->OMSetRenderTargets(1, &rtv, nullptr);

  // G1/X28: the chain is PREMULTIPLIED, so the clear value must be premultiplied
  // too. A straight-alpha colour here is a real bug: with alpha 1.0 the two are
  // numerically identical, so the clear deliberately keeps alpha at 1.0 to stay
  // correct by construction while later M2 work introduces partial alpha.
  const FLOAT clear[4] = {0.16f, 0.22f, 0.28f, 1.0f};
  ctx->ClearRenderTargetView(rtv, clear);

  // The clear must be ordered before the Present. D3D11 context state is not
  // implicitly flushed by Present on every adapter, and an unflushed clear is
  // exactly the kind of thing that presents as a stale or black buffer.
  ctx->Flush();

  // BUG-26: DXGI_PRESENT_DO_NOT_WAIT is right for the frame loop, where a
  // dropped frame is simply retried on the next iteration, but wrong mid-drag,
  // where a dropped frame IS the visible black strip. With allow_wait the call
  // may block until a buffer frees instead of returning WAS_STILL_DRAWING.
  // SyncInterval stays 0 either way: waiting for a vblank inside the modal loop
  // is felt as the window lagging the pointer (X29's split of pacing vs cost).
  const UINT present_flags =
      allow_wait ? 0u : static_cast<UINT>(DXGI_PRESENT_DO_NOT_WAIT);
  hr = dx.Present(present_flags);
  // BUG-02: WAS_STILL_DRAWING means the frame DID NOT reach the screen.
  // It is not a device failure, but it is not success either - the caller
  // must keep the dirty flag set and retry on a later iteration.
  if (hr == DXGI_ERROR_WAS_STILL_DRAWING) return PresentResult::Retry;
  if (hr == DXGI_ERROR_DEVICE_REMOVED || hr == DXGI_ERROR_DEVICE_RESET) {
    // E3: recoverable, but rebuilding device + chain + visual is M0+ work. Report
    // it rather than spinning on a dead device.
    FCS_CHECK_HR(hr);
    return PresentResult::Failed;
  }
  return (FCS_CHECK(hr) == S_OK) ? PresentResult::Presented
                                 : PresentResult::Failed;
}

// BUG-26. Make the swap chain cover the LIVE client rect.
//
// Extracted from RunApp's loop so the synchronous size/move path and the frame
// loop share ONE implementation. Two copies of a resize guard is how the strip
// gets fixed in one path and left broken in the other - this file has already
// been bitten once by two owners of the same composition state (see the
// ownership note on BuildCompositionTree).
// BUG-26: takes the size explicitly rather than reading the client rect, because
// WM_WINDOWPOSCHANGING needs to stage the PROPOSED size before the window rect
// has actually moved to it. Every other caller wants the live client rect and
// goes through EnsureChainCoversClient below.
bool EnsureChainForSize(UINT w, UINT h) {
  if (g_frame.dx == nullptr || g_frame.win == nullptr) return false;
  DxDevice& dx = *g_frame.dx;
  HWND hwnd = g_frame.win->handle();

  if (w == 0 || h == 0) return false;   // E2: never touch a zero-size surface
  if (w == g_frame.last_w && h == g_frame.last_h) return true;

  // E2: on SUCCESS the DComp content link must be re-established.
  // ResizeBuffers allocates a new back buffer but does not re-point root_ at it,
  // so the old (now destroyed) buffer stays composited and the window goes
  // black. Note the polarity: AttachContent is the success path.
  bool resized = dx.ResizeSwapChain(w, h);
  if (resized) {
    dx.AttachContent();
  } else {
    // Recovery. The old code called BuildCompositionTree here, which for an
    // ALREADY-EXISTING chain only re-links content and never changes its size -
    // so it could not possibly fix a size mismatch. A failed resize is only
    // recoverable by rebuilding at the new size.
    dx.DestroySwapChain();
    resized = dx.CreateSwapChain(hwnd, w, h, g_frame.tier);
  }
  // last_w/last_h advance ONLY after the chain really is the new size. Advancing
  // them BEFORE the attempt was a real defect: a single failed ResizeBuffers made
  // the guard permanently false, so the chain stayed at its old size for the
  // rest of the session and the uncovered part of the window stayed black.
  // Caching the ATTEMPT instead of the RESULT is what made the failure permanent.
  if (resized) {
    g_frame.last_w = w;
    g_frame.last_h = h;
  }
  return resized;
}

bool EnsureChainCoversClient() {
  if (g_frame.win == nullptr) return false;
  RECT cr = {};
  if (!::GetClientRect(g_frame.win->handle(), &cr)) return false;
  const UINT cw = static_cast<UINT>(cr.right - cr.left);
  const UINT ch = static_cast<UINT>(cr.bottom - cr.top);
  // BUG-26: mid-drag the chain is deliberately LARGER than the window (see
  // WM_ENTERSIZEMOVE) so the drag costs zero ResizeBuffers. Never shrink it
  // here - that would undo the whole point - and only grow, and only if the
  // user outran the margin.
  if (g_frame.oversize_active && cw <= g_frame.last_w && ch <= g_frame.last_h) {
    return true;
  }
  return EnsureChainForSize(cw, ch);
}

// BUG-26. One synchronous resize-and-present, safe to call from inside the
// system's modal size/move loop when the frame loop cannot run.
//
// The reentrancy guard is load-bearing: Present can dispatch messages, and
// starting a second present from inside the first is how this kind of code
// deadlocks. `in_flight` is cleared on every path.
void SyncResizeAndPresent() {
  if (g_frame.dx == nullptr || g_frame.win == nullptr) return;
  if (g_frame.in_flight) return;
  if (::IsIconic(g_frame.win->handle())) return;   // E2: nothing to draw

  g_frame.in_flight = true;
  // BEFORE the resize, so the line records how far the buffer had fallen behind
  // the window at the moment the compositor would be showing the shortfall.
  if (g_frame.in_size_move && !g_frame.sync_traced) {
    g_frame.sync_traced = true;
    TraceDrag("sync");
  }
  EnsureChainCoversClient();
  const PresentResult pr = PresentFrame(*g_frame.dx, /*allow_wait=*/true);
  g_frame.in_flight = false;

  // Same contract as the loop: only a frame that actually reached the screen
  // consumes the dirty flag, so a Retry is still redrawn by the loop.
  if (pr == PresentResult::Presented) g_frame.win->ClearDirty();
}

// E2/W1 diagnostic sink.
//
// fcs_glass is a WIN32-subsystem binary, so std::printf reaches no console and
// the size-mismatch warning was invisible - which is precisely why a 354 px
// uncovered strip survived two sign-off passes. This appends to trace.txt
// (already in .gitignore) so the numbers are readable after the fact.
void TraceSize(const char* fmt, UINT a, UINT b, UINT c, UINT d) {
  char line[256];
  std::snprintf(line, sizeof(line), fmt, a, b, c, d);
  std::printf("FCS Glass: %s\n", line);
  if (FILE* f = std::fopen("trace.txt", "a")) {
    std::fprintf(f, "%s\n", line);
    std::fclose(f);
  }
}

// BUG-26 diagnostic. ONE line at drag start, at the first synchronous present,
// and at drag end - bounded on purpose for the reason in g_frame.sync_traced.
// The three numbers are the ones that decide the bug: if the buffer lags the
// WINDOW while the window is growing, the shortfall is exactly the region the
// compositor shows dark.
void TraceDrag(const char* tag) {
  if (g_frame.win == nullptr || g_frame.dx == nullptr) return;
  RECT wr = {};
  RECT cr = {};
  ::GetWindowRect(g_frame.win->handle(), &wr);
  ::GetClientRect(g_frame.win->handle(), &cr);
  UINT bw = 0;
  UINT bh = 0;
  (void)g_frame.dx->SwapChainBufferSize(&bw, &bh);
  char line[176];
  std::snprintf(line, sizeof(line), "%-5s window=%ux%u client=%ux%u buffer=%ux%u",
                tag, static_cast<UINT>(wr.right - wr.left),
                static_cast<UINT>(wr.bottom - wr.top),
                static_cast<UINT>(cr.right - cr.left),
                static_cast<UINT>(cr.bottom - cr.top), bw, bh);
  std::printf("FCS Glass: %s\n", line);
  if (FILE* f = std::fopen("trace.txt", "a")) {
    std::fprintf(f, "%s\n", line);
    std::fclose(f);
  }
}

int RunApp(HINSTANCE instance) {
  // J4 first: restrict the DLL search order before anything else can load a DLL.
  if (!InitDllSearch()) {
    std::printf("FCS Glass: DLL search could not be restricted to System32 (J4).\n");
  }

  // P1. The manifest already set the process default; this is the runtime path and
  // is what the probe measures. It must run before any HWND exists.
  const DpiState dpi = InitDpiAwareness();

  const OsInfo os = QueryOsInfo();
  std::printf("FCS Glass M0 - profile %s, build %lu, per-monitor-v2 %s\n",
              os.profile == OsProfile::Fluent ? "Fluent" : "Classic",
              static_cast<unsigned long>(os.build),
              dpi.per_monitor_v2 ? "on" : "off");

  // D5: one device, shared by every window this process owns.
  DxDevice dx;
  if (!dx.Create()) {
    std::printf("FCS Glass: device creation failed (0x%08lX).\n",
                static_cast<unsigned long>(dx.info().last_failure));
    return 1;
  }
  std::printf("FCS Glass: %s device, feature level %x, presented format %d.\n",
              dx.info().driver == DriverKind::Hardware ? "hardware" : "WARP",
              static_cast<unsigned>(dx.info().feature_level),
              static_cast<int>(dx.RenderTargetFormat(QualityTier::Full)));

  static const wchar_t kClass[] = L"FCSGlassHostWindow";
  if (!RegisterHostClass(instance, nullptr, &HostWndProc, kClass)) {
    std::printf("FCS Glass: window class registration failed.\n");
    return 1;
  }

  Window win;
  if (!win.Create(kClass, L"FolderCloneSync", WS_OVERLAPPEDWINDOW, 0,
                  CW_USEDEFAULT, CW_USEDEFAULT, 1280, 800, nullptr, instance)) {
    std::printf("FCS Glass: window creation failed.\n");
    return 1;
  }
  win.SetMinSizeDp(640, 420);   // W6

  // W4: profile attributes, then W1's sheet of glass. Both report HRESULTs.
  win.ApplyProfile(/*fluent=*/os.profile == OsProfile::Fluent, /*dark=*/true);   // P3-09
  if (!win.EnableSheetOfGlass()) {
    std::printf("FCS Glass: sheet of glass not enabled; continuing (reported, not fatal).\n");
  }

  // R6: WARP or a remote session forces Basic. Console + hardware on this host.
  const QualityTier tier =
      (os.session == SessionKind::Remote || dx.info().driver == DriverKind::Warp)
          ? QualityTier::Basic
          : QualityTier::Full;

  if (!BuildCompositionTree(dx, win, tier)) {
    std::printf("FCS Glass: could not build the composition tree.\n");
    return 1;
  }

  // BUG-26: publish the two objects HostWndProc needs to drive a synchronous
  // frame while the system's modal size/move loop owns the thread. Set only once
  // the composition tree exists, so the window procedure can never present
  // against a half-built chain.
  g_frame.dx = &dx;
  g_frame.win = &win;
  g_frame.tier = tier;

  ::ShowWindow(win.handle(), SW_SHOW);
  ::UpdateWindow(win.handle());

  // Research finding 4: present BEFORE the loop, or the window shows empty for one
  // refresh and a later DwmFlush measures the previous frame's interval (X29).
  const PresentResult first = PresentFrame(dx);
  // BUG-02: a dropped first present is retried by the loop below, because
  // the Window starts dirty and stays dirty until a frame actually lands.
  std::printf("FCS Glass: first frame %s; running. Close the window to exit.\n",
              first == PresentResult::Presented ? "presented" : "deferred (will retry)");

  MSG msg = {};
  bool running = true;
  // R6: "render on demand and sleep when idle". The first version of this loop
  // presented unconditionally, which measured 9.2% CPU while doing nothing -
  // against T10's idle budget of under 1%. An unconditional present is a busy
  // wait dressed up as a frame loop, and it also starves WM_SIZE/WM_DPICHANGED
  // processing enough that SetWindowPos was being ignored in testing.
  //
  // The fix is the two-part contract R6 actually states:
  //   1. `needs_redraw` is set by the events that change the image, and cleared
  //      once the frame is presented. Nothing repaints an unchanged window.
  //   2. When there is nothing to draw, block in GetMessageW instead of spinning.
  // BUG-01: dirty state lives on `win` (Window::MarkDirty/ClearDirty) so
  // every message pump - ours AND the system's modal move/size pump - can
  // reach it. The Window starts dirty, so the first frame still happens.
  const int frame_budget_ms = dx.TargetFrameMs(tier);   // R6: 60 fps Full, 30 Basic
  // BUG-26: the size the chain was last successfully sized to now lives on
  // g_frame, because the synchronous size/move path in HostWndProc must share it
  // with this loop. Seeded to 0 so the first frame always performs its initial
  // resize/attach.
  g_frame.last_w = 0;
  g_frame.last_h = 0;
  // E2/W1. Edge-triggered so one bad frame does not spam the log every redraw.
  bool reported_size_mismatch = false;
  // One-shot: the first presented frame's sizes are logged exactly once.
  bool logged_first_size = false;
  // P3-02: consecutive failed presents; bounds the device-rebuild attempts below.
  int present_failures = 0;

  while (running) {
    if (win.dirty()) {
      // Drain input and window messages first, so a resize or DPI change is
      // applied before the frame that depends on it is drawn.
      while (::PeekMessageW(&msg, nullptr, 0, 0, PM_REMOVE)) {
        if (msg.message == WM_QUIT) { running = false; break; }
        ::TranslateMessage(&msg);
        ::DispatchMessageW(&msg);
      }
      if (!running) break;
      // C3: the freeze. Once per frame, AFTER the message drain and BEFORE
      // anything reads input - that ordering is the whole requirement, since two
      // widgets reading input at different points must not be able to disagree
      // because a WM_MOUSEMOVE landed between them. Nothing reads it yet (the
      // widgets arrive with M1's UI layer), but the call belongs here rather than
      // at the first widget: the discipline is only worth anything in place before
      // something depends on it.
      g_frame.input.Snapshot();
      if (!win.ShouldRender()) { win.ClearDirty(); continue; }  // E2 minimized

      RECT cr = {};
      if (::GetClientRect(win.handle(), &cr)) {
        const UINT w = static_cast<UINT>(cr.right - cr.left);
        const UINT h = static_cast<UINT>(cr.bottom - cr.top);
        // E2/BUG-26: the resize itself lives in EnsureChainCoversClient, which
        // the synchronous size/move path also calls - ONE implementation, so the
        // two paths cannot drift apart. It still only touches the chain when the
        // size ACTUALLY changed: calling ResizeSwapChain unconditionally every
        // frame reallocated both buffers on each redraw for no reason, a resize
        // storm that also drops the DComp content link (see AttachContent).
        EnsureChainCoversClient();
        // One-shot: record the first presented frame's geometry, because the
        // first question about any fill bug is "what did we ask for vs what did
        // the client actually give us".
        if (!logged_first_size) {
          logged_first_size = true;
          RECT w0 = {};
          ::GetWindowRect(win.handle(), &w0);
          TraceSize("WINDOW %ux%u", static_cast<UINT>(w0.right - w0.left),
                    static_cast<UINT>(w0.bottom - w0.top), 0, 0);
          UINT bw0 = 0, bh0 = 0;
          if (dx.SwapChainBufferSize(&bw0, &bh0)) {
            TraceSize("FIRST buffer=%ux%u client=%ux%u", bw0, bh0, w, h);
          }
        }
        // E2/W1. The back buffer must COVER the client rect. The cached
        // width_/height_ only record what we ASKED for, so they cannot detect a
        // resize that was skipped or silently failed - which is exactly how the
        // M0 capture ended up with an uncovered margin compositing black.
        // Read the granted size back from DXGI and compare it to the live client
        // rect. On a PREMULTIPLIED chain the uncovered region is alpha 0, so it
        // shows whatever is behind the window instead of our clear colour.
        {
          UINT bw = 0;
          UINT bh = 0;
          if (dx.SwapChainBufferSize(&bw, &bh)) {
            // BUG-26: mid-drag the chain is intentionally LARGER than the client,
            // so comparing the two is meaningless until the drag lands.
            if (!g_frame.in_size_move && (bw != w || bh != h)) {
              if (!reported_size_mismatch) {
                reported_size_mismatch = true;
                TraceSize("MISMATCH buffer=%ux%u client=%ux%u", bw, bh, w, h);
                // Force another resize attempt next frame. last_w/last_h are
                // deliberately not advanced while the sizes disagree.
                g_frame.last_w = 0;
                g_frame.last_h = 0;
              }
            } else {
              reported_size_mismatch = false;
            }
          }
        }
        const PresentResult pr = PresentFrame(dx);
        if (pr == PresentResult::Retry) {
          // BUG-02: the present did not happen; keep the dirty flag set
          // and retry next iteration. The 1 ms yield stops this becoming a
          // busy spin inside the single vblank it usually takes.
          ::Sleep(1);
          continue;
        }
        if (pr == PresentResult::Failed) {
          // E3: the chain or device may be gone. Rebuild; if that also
          // fails, drop the dirty flag so a dead device cannot spin the
          // loop - the next event re-arms a frame.
          // P3-02: bound the recovery. A persistent NON-device failure (bad descriptor, an
          // AcquireRtv error) must not rebuild the whole device on every iteration for
          // ever. Give up for this dirty cycle; the next event re-arms a fresh burst.
          if (++present_failures > 3) {
            TraceSize("PRESENT_GIVEUP failures=%u 0 0 0", static_cast<UINT>(present_failures), 0, 0, 0);
            present_failures = 0;
            win.ClearDirty();
            continue;
          }
          dx.Recreate();
          g_frame.last_w = 0;   // P3-02: the rebuilt chain is a new size authority
          g_frame.last_h = 0;
          if (BuildCompositionTree(dx, win, tier)) {
            // BUG-20: the failed present never reached the screen, and a
            // freshly built chain has never been presented - it holds
            // whatever the allocator left, which composites as black. Keep
            // the dirty flag SET so the next iteration presents the
            // recovered chain, instead of idling on a black window until
            // some unrelated event re-arms a frame. Sleep one frame budget
            // so a persistently failing present cannot spin hot.
            ::Sleep(static_cast<DWORD>(frame_budget_ms));
            continue;
          }
          FCS_CHECK_HR(dx.info().last_failure);
        }
        if (pr == PresentResult::Presented) present_failures = 0;   // P3-02
        win.ClearDirty();
      }
      // R6 frame cap. Measured against the monotonic clock rather than assumed;
      // DwmFlush is deliberately NOT used here because it measures the refresh
      // period, not this frame's cost (X29).
      ::Sleep(static_cast<DWORD>(frame_budget_ms));
      continue;
    }

    // Idle: block until a message arrives instead of spinning. This is the
    // difference between ~0% and ~9% CPU.
    //
    // RV-06: GetMessageW has THREE outcomes, not two. `> 0` is a message, 0 is
    // WM_QUIT, and -1 is an ERROR. Testing `> 0` sends -1 straight back into the
    // pump, which returns -1 again immediately - an idle spin at 100% CPU, in the
    // one place whose entire job is to not spin. Unreachable today (null filter,
    // one thread) and live the moment the pump grows an hwnd filter or a second
    // window, which is M1's dock.
    //
    // Note WM_QUIT is also now taken from the RETURN VALUE rather than by
    // inspecting msg.message. When GetMessage returns 0 the struct is not
    // guaranteed to be meaningful, so reading it was a second, quieter version of
    // the same mistake.
    switch (::GetMessageW(&msg, nullptr, 0, 0)) {
      case -1: {
        // LastError is captured before anything else can clobber it.
        const DWORD err = ::GetLastError();
        // TraceSize is a fixed 5-argument printf, so the unused slots must be
        // passed explicitly - reading them is undefined, not zero.
        TraceSize("PUMP_ERROR GetMessageW failed GetLastError=%lu 0 0 0",
                  static_cast<UINT>(err), 0, 0, 0);
        // Leave the loop rather than spin. A pump that cannot read its queue has
        // no way to learn that WM_QUIT is coming, so idling on it forever would
        // strand the process with no way out.
        running = false;
        break;
      }
      case 0:
        running = false;
        break;
      default:
        ::TranslateMessage(&msg);
        ::DispatchMessageW(&msg);
        // BUG-01: the stale-image flags are set inside HostWndProc
        // (Window::MarkDirty) for every message that changes the image, so
        // no per-message bookkeeping is needed here - and messages
        // dispatched by OTHER pumps (modal resize, menus) are covered too.
        break;
    }
  }

  // BUG-26: unpublish before the objects die. Teardown itself sends messages
  // (WM_SIZE/WM_DESTROY), and a synchronous present against a destroyed chain is
  // exactly the crash this ordering prevents.
  g_frame.dx = nullptr;
  g_frame.win = nullptr;

  dx.DestroySwapChain();
  win.Destroy();
  return 0;
}

}  // namespace
}  // namespace fcs

// WIN32 subsystem (CMakeLists sets WIN32), so the entry point is wWinMain. The
// printf calls above reach no console in a normal run; they exist for a debugger
// and for the M0 harness, which attaches a console. The exit code is what a
// script can read.
int APIENTRY wWinMain(HINSTANCE instance, HINSTANCE, LPWSTR, int show) {
  (void)show;
  return fcs::RunApp(instance);
}