fix(recording): stop macOS fragments carrying an offset the box cannot hold - #375
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…t hold a6795d2 gave macOS the same crash-resilience Windows got, in one line: movieFragmentInterval. On macOS that line destroyed every recording it touched. Capture stopped after a few seconds while the HUD counted on, and stop answered AVFoundationErrorDomain -11800 / -16341, so the take was discarded: no sidecars, no editor. Six takes on the shipped rc.1 lost ~530 MB of perfectly decodable video between them. The container was never the problem, and neither were the timestamps -- every sample file has strictly monotonic DTS. What is wrong is in the fragment bytes: each `trun` goes out version 0 carrying composition offsets like 0xFFFFFFF6, which is -10 reinterpreted, because ISO/IEC 14496-12 8.8.8.2 defines that field as unsigned in version 0 and signed only in version 1. Offsets are negative only because the encoder reorders frames, and it reorders because AVVideoAllowFrameReorderingKey is never set, so it runs High profile with has_b_frames=2. MediaToolbox raises -16341 from exactly one site -- inside the function that writes moof/mfhd/traf/trun -- which is why the failure needs movieFragmentInterval to exist at all and always lands on a fragment boundary: the two audio failures hit at 1.0s and 2.0s against a 1s interval. Turning reordering off makes every offset zero and PTS == DTS, and the fragment becomes representable. A screen recorder pays nothing for it -- B-frames buy compression on lookahead-friendly content and cost encode latency, the wrong trade for real-time capture. Measured on macOS 26.5 / M1, 1080p30 with system audio, the configuration that kills the current build in 1-2s: clean stop at 43.66s, has_b_frames 2 -> 0, 0 of 819 packets with pts != dts. SIGKILL at 25s leaves 27 moof, decodes clean (ffmpeg -v error -f null - exit 0) and recovers 28.01s with both tracks. So the recording survives AND the crash-resilience the commit existed for now actually works on macOS, which it never did. The second change is why this cost a whole recording to learn one bit. A failed AVAssetWriter keeps accepting appends and keeps answering false; the helper discarded that Bool after the first frame and read writer.status only in finishWriter(). That is the entire reason the HUD counted to 02:02 over a writer that died at 00:04. The Windows helper checks every WriteSample HRESULT and escalates; this reports once, at the append that failed, carrying the live writer.error. It does not abort the capture -- handlers.ts tears its error listener down once recording-started arrives, so acting on this mid-recording is a TypeScript change and belongs in its own commit.
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: defaults Review profile: CHILL Plan: Pro Plus Run ID: 📒 Files selected for processing (1)
📝 WalkthroughWalkthroughThe recorder now reports video writer failures once, includes writer diagnostics, emits ChangesScreen capture writer behavior
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to The recording helper may report a capture failure for an append that did not actually fail the writer, while missing failures that occur when the input is temporarily unavailable; this could cause premature rejection or delayed error reporting. The PR is mergeable with explicit owner follow-up to gate reporting on the writer’s failed status. Sequence Diagram(s)sequenceDiagram
participant ScreenCaptureRecorder
participant AVAssetWriter
participant RecordingEvents
ScreenCaptureRecorder->>AVAssetWriter: append video sample
AVAssetWriter-->>ScreenCaptureRecorder: append result
ScreenCaptureRecorder->>RecordingEvents: recording-started on success
ScreenCaptureRecorder->>RecordingEvents: writer-failed-during-capture with diagnostics on failure
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The Results log is the durable artifact, and a row that records a blocker with no pointer to its resolution invites the next person to re-derive it. #375 root-causes this one to a version 0 trun carrying a negative composition offset, and the row now says so -- along with the part that still needs doing, which is re-running this section against a CI build that carries the fix.
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In
`@electron/native/screencapturekit/Sources/OpenScreenScreenCaptureKitHelper/ScreenCaptureRecorder.swift`:
- Around line 323-346: Update finishWriter() so its final writer-failure branch
calls reportWriterFailure("writer finalization") instead of emitting
writer-failed directly, preserving the reporter’s one-time suppression behavior.
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Configuration used: defaults
Review profile: CHILL
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Run ID: afc810d4-2468-4855-8b79-9c4669bec8a6
📒 Files selected for processing (1)
electron/native/screencapturekit/Sources/OpenScreenScreenCaptureKitHelper/ScreenCaptureRecorder.swift
… rate Review caught that reportWriterFailure and finishWriter both emitted `writer-failed`, and proposed routing finalization through the one-time reporter. That would break stopping. handlers.ts settles the stop promise on exactly one of `recording-stopped` or `writer-failed`, so suppressing the terminal event whenever an append already fired turns every writer failure into the "Saving..." hang instead of an error -- the exact symptom this branch exists to remove. The two sites answer different questions, so they now carry different codes: `writer-failed-during-capture` says when the writer died, `writer-failed` says whether stopping worked. Verified by putting the bug back and watching a failing run emit exactly one of each. Rebuilding that broken variant also corrected the evidence. It survived 22.2s at 30 fps, where the same configuration had failed twice at 1-2s, so the failure is probabilistic and my "2/2 versus 3/3" was a sample, not a law. It is rate-dependent: at ~57 fps, the rate the app drives and the rate at which the shipped binary failed 6/6, reordering on dies at 13.0s and reordering off stops clean at 31.6s. The comment now quotes the frame rate beside every number, because a reproduction that is only sometimes reproducible is exactly the kind a future reader will try once, fail to trigger, and conclude was never real. The case for the fix does not rest on those counts. It rests on the bytes: the composition offsets are unrepresentable in a version 0 trun in every fragmented file, whether or not that particular run happened to die.
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Review pushed on four sentences, and rebuilding the broken arm while answering it turned one of them from overstated into wrong. "Not load-related" was drawn from two standalone reproductions at a lower resolution. Those show the failure is not confined to the app's 4K60 path, which is not the same thing: append rate demonstrably changes how reliably it bites, reliably at ~57 fps and intermittently at 30. "A/B isolates it" was a sample presented as a law. A later rebuild of the with-the-line arm survived 22.2s at settings that had killed it twice at 1-2s, so the counts narrow the with-audio path and no more. The case rests on the bytes, not the tally, and the row now says so. The same variable also dissolves the video-only local-versus-shipped gap this row called unexplained: 56.6 fps shipped against 29 fps locally, not the released artifact. "Duration exact" was followed in the same clause by the 7 ms it differed by. "Root-caused and fixed in #375" claimed for this run a validation it never did. The run reproduced the failure; the fix is verified at helper level in #375 and in the packaged app nowhere yet. Also attributes the mvex/moof observations to the samples they came from, including the one kill that carries mvex with zero moof because capture had already stalled twelve seconds before the kill landed.
EtienneLescot
added a commit
that referenced
this pull request
Aug 14, 2026
The Results log is the durable artifact, and a row that records a blocker with no pointer to its resolution invites the next person to re-derive it. #375 root-causes this one to a version 0 trun carrying a negative composition offset, and the row now says so -- along with the part that still needs doing, which is re-running this section against a CI build that carries the fix.
EtienneLescot
added a commit
that referenced
this pull request
Aug 14, 2026
Review pushed on four sentences, and rebuilding the broken arm while answering it turned one of them from overstated into wrong. "Not load-related" was drawn from two standalone reproductions at a lower resolution. Those show the failure is not confined to the app's 4K60 path, which is not the same thing: append rate demonstrably changes how reliably it bites, reliably at ~57 fps and intermittently at 30. "A/B isolates it" was a sample presented as a law. A later rebuild of the with-the-line arm survived 22.2s at settings that had killed it twice at 1-2s, so the counts narrow the with-audio path and no more. The case rests on the bytes, not the tally, and the row now says so. The same variable also dissolves the video-only local-versus-shipped gap this row called unexplained: 56.6 fps shipped against 29 fps locally, not the released artifact. "Duration exact" was followed in the same clause by the 7 ms it differed by. "Root-caused and fixed in #375" claimed for this run a validation it never did. The run reproduced the failure; the fix is verified at helper level in #375 and in the packaged app nowhere yet. Also attributes the mvex/moof observations to the samples they came from, including the one kill that carries mvex with zero moof because capture had already stalled twelve seconds before the kill landed.
EtienneLescot
added a commit
that referenced
this pull request
Aug 14, 2026
The Results log is the durable artifact, and a row that records a blocker with no pointer to its resolution invites the next person to re-derive it. #375 root-causes this one to a version 0 trun carrying a negative composition offset, and the row now says so -- along with the part that still needs doing, which is re-running this section against a CI build that carries the fix.
EtienneLescot
added a commit
that referenced
this pull request
Aug 14, 2026
Review pushed on four sentences, and rebuilding the broken arm while answering it turned one of them from overstated into wrong. "Not load-related" was drawn from two standalone reproductions at a lower resolution. Those show the failure is not confined to the app's 4K60 path, which is not the same thing: append rate demonstrably changes how reliably it bites, reliably at ~57 fps and intermittently at 30. "A/B isolates it" was a sample presented as a law. A later rebuild of the with-the-line arm survived 22.2s at settings that had killed it twice at 1-2s, so the counts narrow the with-audio path and no more. The case rests on the bytes, not the tally, and the row now says so. The same variable also dissolves the video-only local-versus-shipped gap this row called unexplained: 56.6 fps shipped against 29 fps locally, not the released artifact. "Duration exact" was followed in the same clause by the 7 ms it differed by. "Root-caused and fixed in #375" claimed for this run a validation it never did. The run reproduced the failure; the fix is verified at helper level in #375 and in the packaged app nowhere yet. Also attributes the mvex/moof observations to the samples they came from, including the one kill that carries mvex with zero moof because capture had already stalled twelve seconds before the kill landed.
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Fixes the macOS half of
a6795d23. Test results that found it: #374.What was broken
a6795d23gave macOS the same crash-resilience Windows got, in one line:movieFragmentInterval. On macOS that line destroyed recordings. Capture stopped while the HUD counted on, and stop answeredAVFoundationErrorDomain -11800/-16341, so the take was discarded — no sidecars, no editor. Six takes on the shipped rc.1 lost ~530 MB of decodable video between them.Root cause
Not the container, and not the timestamps — every sample file has strictly monotonic DTS. The defect is in the fragment bytes:
trungoes out version 0 carrying composition offsets like0xFFFFFFF6, which is −10 reinterpretedAVVideoAllowFrameReorderingKeyis never set — so it runs High profile withhas_b_frames=2-16341is emitted from exactly one site in MediaToolbox, inside the function that writesmoof/mfhd/traf/trun. Hence the failure requiresmovieFragmentIntervalto exist at all, and lands on fragment boundaries — the two audio failures hit at 1.0 s and 2.0 s against a 1 s interval.Turning reordering off makes every offset zero and
PTS == DTS, so the fragment becomes representable. A screen recorder pays nothing for it: B-frames buy compression on lookahead-friendly content and cost encode latency, the wrong trade for real-time capture.Measured
macOS 26.5 / M1, 1080p with system audio. The failure is probabilistic and scales with append rate, so the frame rate is quoted with every number:
Byte-level, on the fixed build:
has_b_frames2 → 0,pts != dts0 / 1794.Crash-resilience actually works now, which it never did on macOS: SIGKILL at 25 s leaves 27
moof, decodes clean (ffmpeg -v error -f null -exit 0) and recovers 28.01 s with both tracks.Second change
A failed
AVAssetWriterkeeps accepting appends and keeps answeringfalse. The helper discarded that Bool after the first frame and readwriter.statusonly infinishWriter()— the entire reason the HUD counted to02:02over a writer that died at00:04. The Windows helper checks everyWriteSampleHRESULT and escalates (mf_encoder.cpp:1551,main.cpp:999-1007).The two reporting sites carry different codes on purpose:
writer-failed-during-capture— diagnostic: when the writer diedwriter-failed— terminal: whether stopping workedhandlers.ts:1487-1493settles the stop promise on exactly one ofrecording-stoppedorwriter-failed. Giving both sites one code behind a one-shot guard would mean a writer that died mid-capture emits nothing at stop, so the promise never settles and every failure becomes the "Saving…" hang. Verified by putting the bug back: a failing run emits exactly one of each.It deliberately does not abort the capture — handlers.ts tears its error listener down once
recording-startedarrives, so acting on this mid-recording is a TypeScript change for its own commit.Does this affect Windows?
No. Windows already enforces the invariant macOS lacked, twice independently —
main.cpp:999-1003andmf_encoder.cpp:573-575— and its audio is a synthesized sample counter, not a device clock.Not covered
/Applications, so the helper could not be swapped in place. Verification is at helper level, driven from the shell with the exact JSON contract the app sends. A CI build should re-run the checklist's recording section before rc.2 ships.has_b_frames == 0andmoof > 0after a kill would be cheap, alongside the existing truncation test.