fix(windows): repeat the owed screen slots so a loaded writer catches up

[EtienneLescot]

Summary

#1056 made a late Windows writer catch up by running ticks back to back, one frame each. A tick still read the screen back whenever WGC had a new frame, and only an unchanged screen repeated for free. Under load a readback (staging CopyResource + Map, then BGRA to NV12 on the CPU) can cost more than the 16.7 ms period. On a changing screen the catch-up ticks then cost as much as the ticks that fell behind, the count never catches up, and the video plays earlier and earlier against the audio. The v2.1.0 candidate lost 217 ms that way over a 10 min take; the webcam, which needs only 30 of those ticks a second, stayed level with the audio.

• FrameSlotClock::isBehind(now): the slot about to be written is already past, because the one after it is due too.
• The writer fills a past slot with repeatLastVideoSample (no readback), changed screen or not. Only the live slot reads back, so the newest picture lands in the slot it belongs to and catching up costs an encode per slot, not a readback.
• PictureReadback keeps a change seen on a past slot pending until a live slot reads it back, since WGC sends no new frame for a screen that then stays still.
• Unchanged: the DXGI input path (opt-in, no repeat support), the webcam cadence, stop latency.

Related issue

Fixes #1148

Candidate for cherry-pick to release/v2.1.0.

Type of change

• Bug fix
• Feature
• Enhancement
• Documentation
• Refactor / maintenance
• Performance
• Security

Release impact

• Patch
• Minor
• Major / breaking change
• No release note needed

Desktop impact

• Windows
• macOS
• Linux
• Installer / packaging
• Not platform-specific

Screenshots / video

None (capture timing).

Testing

• frame_slot_clock_test (run by scripts/build-windows-wgc-helper.mjs) gains a model of the loaded writer on a screen that changes every tick: 10 min at 60 fps, 20 ms readback, 4 ms repeat. Reading back on every catch-up tick writes 30000 frames (100 s of video played early); repeating the past slots writes all 36000, and no picture leads its slot by more than a period. A 17 ms readback, just over budget, also keeps the full 36000. Plus isBehind edge cases (nothing owed before the first frame, live slot not past) and PictureReadback cases (a change on a past slot followed by a live tick with no new frame is read back once).
• Built and run here with clang++ on macOS (the header and test are portable); the Windows helper itself was not built on this machine, the main.cpp change is a condition and comments.
• No TS/JS change, npm run lint clean.

Manual check left: on Windows, a 10 min take with webcam and system audio under a CPU/GPU load (an export loop), read a filmed clock or a clap at start and end, and compare ffprobe -show_entries stream=start_time,duration across the screen, audio and webcam streams. The helper's [pacing] frames=... elapsed_ms=... line at stop should give frames/elapsed equal to the fps.

Summary by CodeRabbit

• Improvements
  • Video capture repeats the most recently encoded picture for overdue frame slots instead of waiting for a new screen readback. This keeps video timing consistent when capturing a new picture takes longer than expected.
  • Screen changes remain available for a current frame slot, so the latest picture can be captured after overdue slots are repeated. This also limits how far a picture leads its intended slot.

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Reviewing files that changed from the base of the PR and between 2d03271 and 7da42e7.

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📒 Files selected for processing (3)

• electron/native/wgc-capture/src/frame_slot_clock.h
• electron/native/wgc-capture/src/frame_slot_clock_test.cpp
• electron/native/wgc-capture/src/main.cpp

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📝 Walkthrough

📝 Walkthrough

Walkthrough

The WGC frame clock now reports when its next slot is due. The screen writer repeats the previous sample for overdue slots, even when the screen changes. The new picture remains pending for a live slot. Tests compare frame counts and picture lead during loaded capture.

Changes

WGC frame catch-up

Layer / File(s)	Summary
Clock and readback decisions electron/native/wgc-capture/src/frame_slot_clock.h, electron/native/wgc-capture/src/frame_slot_clock_test.cpp	FrameSlotClock adds isBehind. PictureReadback tracks pending screen changes and skips readback when a slot is overdue. Tests cover the readback decisions and loaded capture frame counts.
Repeat samples for overdue slots electron/native/wgc-capture/src/main.cpp	The writer repeats the previous sample when a slot is overdue, even if the picture changed. The new picture remains pending for a live slot.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant writeVideoFrames
  participant FrameSlotClock
  participant PictureReadback
  participant VideoSample
  writeVideoFrames->>FrameSlotClock: Check whether the next slot is due
  writeVideoFrames->>PictureReadback: Check readback eligibility
  alt Slot is overdue
    writeVideoFrames->>VideoSample: Repeat the previous sample
    Note over writeVideoFrames,PictureReadback: Keep the changed picture pending
  else Slot is live
    writeVideoFrames->>PictureReadback: Read back the pending picture
    writeVideoFrames->>VideoSample: Write the current sample
  end

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Merge Risk: ⚪ Minimal · up to 7da42

The previously reported risk of losing a screen change is addressed. No actionable merge blocker remains after normal checks.

Pre-merge checks | 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check		Issue #1148 requires the Windows WGC CPU-readback screen track to keep its nominal frame count under load. FrameSlotClock::isBehind identifies overdue slots. The Windows writer repeats the previous …
Out of Scope Changes check		The changes are limited to the frame clock and readback model, related automated tests, and the Windows WGC writer. These changes directly support issue #1148. The summary reports no changes to the DX…
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files.
Title check		The title clearly identifies the Windows pacing fix: repeating overdue screen slots so a loaded writer can catch up.
Description check		The description is complete and follows the repository template. It explains the problem, implementation, issue reference, release and platform impact, testing, and remaining manual validation.

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[EtienneLescot]

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @electron/native/wgc-capture/src/main.cpp:
- Line 1294: Update the frame-handling logic around usesDxgiInput and
captureVideoSample to retain a pending picture-change flag across repeated or
empty slots. Clear the flag only after captureVideoSample captures that picture,
and add a test covering a change on an overdue tick followed by an empty live
tick.

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📒 Files selected for processing (3)

• electron/native/wgc-capture/src/frame_slot_clock.h
• electron/native/wgc-capture/src/frame_slot_clock_test.cpp
• electron/native/wgc-capture/src/main.cpp

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[EtienneLescot]

#1155 and #1165 target the same 217 ms loss (#1148 and #1164 are the same report). #1165 is the root fix; #1155's premise does not hold on the measured file.

What the measured file says (recording-1791818829985.mp4, the v2.1.0 candidate take):

• The frame count is not short. 37,539 frames is 625.65 s at 60 fps, the audio's length (625.643 s). The catch-up from fix(windows): keep the screen video on the clock when the writer misses ticks #1056 kept up; no debt was left unpaid, so fix(windows): repeat the owed screen slots so a loaded writer catches up #1155's throughput cause is not what happened in this take.
• The encoder does not renumber frames at the nominal rate. Inside each fragment the trun durations are 858 / 1072 / 1286 units of 1/60000 s, the real intervals. The hardware and software encoders, fragmented and plain MP4, all keep sample times (measured in Windows: screen video falls behind audio on long takes (fragmented MP4 drops the gap at fragment ends) #1164).
• The time is lost at fragment ends. Every fragment ends on exactly 1000 units (1/60 s) whatever the real interval: MFCreateFMPEG4MediaSink takes the last sample's own nominal SampleDuration, and the next fragment's tfdt follows on. Around 400 s every fragment reads … 1072, 1072, 1000: 1.2 ms lost a second while the desktop updates at 56 Hz. An instrumented build shows the file departing from the stamped times only at samples 61, 122, 183….

What #1165 does: each sample gets its real duration (held until the next one arrives), every sample is stamped with its tick's time on the recording clock, and a late tick leaves a correctly timed gap instead of a burst.

Measured under load, 10 min, both helpers recording at once: RC helper video 599.739 s vs audio 600.043 s (-303 ms); #1165 600.034 s vs 600.043 s (-8 ms).

#1155 would make catch-up cheaper, but with #1165 there is no catch-up left to make cheaper: a late tick leaves a gap that plays at the right time. Not closing #1155 from here since another session opened it; the maintainer decides.