fix(moq-video): print GPU-composited windows instead of copying a black DC - #3370
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WalkthroughThe Windows capture backend now identifies windows through process id and creation time instead of a writable window property. It probes responsiveness before calling Merge Risk: 🟠 High · up to Windows capture can switch to an unselected same-process replacement window or become stuck during capture and shutdown. These correctness, privacy, and availability issues should be fixed before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 35.71% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 14 functions across 1 files. (3 skipped: 3 unsupported.)
✨ Finishing Touches✨ Simplify code
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Actionable comments posted: 2
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Inline comments:
In `@rs/moq-video/src/capture/window.rs`:
- Around line 258-266: Update WindowIdentity::matches to reject an HWND
recreated by the same process instead of treating matching process and
creation-time data as sufficient; terminate capture when the original target
window is destroyed, or compare a per-window-instance discriminator that changes
across recreation, while preserving the existing successful-read behavior.
- Line 387: Update the capture flow around PrintWindow and PumpGuard so a
synchronous PrintWindow call cannot block the capture pump or prevent shutdown
after responding succeeds. Replace it with a non-blocking/cancellable capture
path, or move frame delivery through a mechanism that can be cancelled before
PumpGuard::drop joins the pump.
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📒 Files selected for processing (7)
quest/m0/README.mdquest/m0/capture-window-lifecycle.mdquest/m0/windows-window-capture-blank.mdquest/m0/x11-window-identity.mdquest/m2/capture-windows.mdrs/moq-video/Cargo.tomlrs/moq-video/src/capture/window.rs
💤 Files with no reviewable changes (3)
- quest/m0/README.md
- quest/m0/capture-window-lifecycle.md
- quest/m0/windows-window-capture-blank.md
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Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
…ck DC `capture/window.rs` took its pixels with `BitBlt` from `GetWindowDC` and then hand-sent `WM_PRINT` to the window. A window composed by DirectComposition (Chrome, Edge, Electron, every UWP app) puts nothing in its window DC, so the copy is black, and a hand-sent `WM_PRINT` is not a documented cross-process contract and does not reach that content either. Call `PrintWindow(hwnd, dc, PW_RENDERFULLCONTENT)` over the existing `BitBlt`. Layering rather than replacing keeps the order the old `BitBlt` + `WM_PRINT` used, so a window that declines to print is no worse off than before. The bounded `SendMessageTimeoutW` becomes a `WM_NULL` responsiveness probe in front of `PrintWindow`, which has no timeout of its own and would otherwise let a hung window wedge the pump thread's join. The per-capture latch that disabled printing after one failure is gone: it is the thing that would leave a composited window black for the rest of a capture. `WindowIdentity` also stops writing to a foreign window. `SetPropW` is refused by UIPI on a higher-integrity window, so `Capture::open` failed outright on an elevated window that plain GDI could have read. Identify the window by its owning process id plus that process's creation time instead, both read-only. Unverified: PR CI is Linux-only and `just rs windows` must run on Windows. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Keep the per-window property marker for ordinary windows, and fall back to process identity only when UIPI rejects the marker for a higher-integrity target.\n\nCo-Authored-By: GPT-5.6-sol <noreply@openai.com>
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| if !responding(handle) { | ||
| return; | ||
| } | ||
| let _ = unsafe { PrintWindow(handle, target, PRINT_WINDOW_FLAGS(PW_RENDERFULLCONTENT)) }; |
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This bug fix changes the Windows-only rendering path and retires the tracking quest without adding a test that proves a DirectComposition target produces non-black, updating frames. Since ordinary CI does not execute this backend and the Windows recipe only compiles it, future changes to the flag or print sequencing can silently restore the original failure; add a Windows regression that fails with the previous WM_PRINT path and exercises this PrintWindow(PW_RENDERFULLCONTENT) call. (Written by GPT-5.6 Sol)
AGENTS.md reference: AGENTS.md:L152-L152
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Root cause
Windows window capture copied pixels from
GetWindowDCand then sentWM_PRINTcross-process. Neither path reaches content rendered through DirectComposition, so Chrome, Edge, Electron, and UWP windows produced black frames.Separately,
WindowIdentityrequiredSetPropWon a foreign window. UIPI rejects that write for a higher-integrity target, so opening an elevated window failed even when GDI could capture it.Summary
BitBltcopy withPrintWindow(hwnd, dc, PW_RENDERFULLCONTENT). A target that declines to print leaves the GDI copy intact.WM_NULLbefore callingPrintWindow, so an already-unresponsive or cross-integrity target skips the synchronous render attempt.Win32_Storage_Xpsmetadata feature required by the Windows bindings forPrintWindow.Public API changes
None. All implementation changes are private to
moq_video::capture::window.Verification
windows0.62 bindings forx86_64-pc-windows-msvc.cargo fmt --all --check: passed after the final rebase.cargo run --locked -p quest -- check: 249 documents passed after the adjacent X11 quest merged.Known platform limitation
PrintWindowis synchronous and Windows exposes no timeout or cancellation form. TheWM_NULLprobe avoids calling it for a target that is already unresponsive, but cannot eliminate the race where a target hangs insidePrintWindow. Capture runs on its dedicated pump thread rather than the async runtime. Replacing this path with Windows.Graphics.Capture, including its consent and WinRT requirements, remains a separate decision in/quest/m2/capture-windows.md.(written by GPT-5.6-sol)