fix dropped and late audio in macos recordings

[Kevin-Liu-01]

my recordly recordings kept coming out with garbled audio, so i dug into why.

each bar is one of my recordings in 1.4.0. red is how much of the system audio is missing, anywhere from 0.6% to 72%. the last bar was recorded with this branch.

what was going on

macos hands recordly audio in small chunks, many times a second, and recordly passes each chunk to an aac encoder. when the encoder was busy, recordly threw the chunk away. AVAssetWriter then joined the chunks that were left end to end, so the audio came out shorter than the video, clicked at every join, and drifted further out of sync the longer i recorded.

window recordings were the worst, because the slow per-frame window crop ran on the same queue as the audio and held it up.

a test app played a rising tone while each build recorded a busy window. upstream lost 648 ms of it in 14 seconds. this branch lost 2 ms.

what this changes

• audio gets its own queue, so video work can't hold it up
• every chunk lands at its own timestamp. gaps become silence and overlaps get trimmed, so audio and video end together
• the .m4a files are written synchronously, so a busy encoder never costs any audio
• mic audio gets converted to 48 khz stereo, since lots of mics give mono at 16 or 24 khz
• pause and resume use the system clock, so audio right after a resume is kept even if the screen hasn't changed yet
• a still screen keeps recording until you hit stop. before, the video ended about 2 s after the last screen change and cut off narration over a still slide
• the media server serves .m4a, so the editor can load these files ([Bug]: macOS mic sidecar (.m4a) is blocked by the media allowlist, so #912 still reproduces #1016)
• exports drop the 2112 priming samples every aac file starts with. keeping them put exported audio 44 ms behind the video, even though the preview was in sync

i checked exports with a test window that flashes and beeps at the same moment. exported audio now matches the recording within 1 ms, and a 5.5 minute export of a real recording lines up with its source everywhere i checked.

how to check

• npx vitest run electron/native src/lib/exporter. on macos this also compiles the swift audio track and checks gaps, overlaps and mono mics
• record a few minutes with system audio and the mic on, then compare lengths with ffprobe -v error -show_entries stream=codec_type,duration on the .mp4 and the .m4a files. on 1.4.0 the audio is shorter. on this branch they match within a frame
• the helper binaries aren't in this pr. npm run build:native-helpers builds them

related

• fixes [Bug]: macOS mic sidecar (.m4a) is blocked by the media allowlist, so #912 still reproduces #1016, and the drift in Delay between the audio and the video #809 (the second comment there found the same cause)
• No audio in final recording on macOS – mic is recorded but not included in the MP4 #1077 looks like the same .m4a problem
• fix(macos): keep microphone audio in sync by gap-filling dropped buffers #946 and fix(macos): preserve audio/video sync during capture #1072 fill the gaps that dropped chunks leave. this pr stops dropping them
• fix: allow .m4a local media so macOS mic sidecars play #1026 and fix(macOS): allow .m4a mic/system audio sidecars through the media allowlist #1041 make the same one-line .m4a fix. i'll drop mine if one of them lands first
• this pr and sharper, smoother window recordings on macos #1089 both touch ScreenCaptureKitRecorder.swift, so i'll rebase whichever lands second

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved macOS recordings by keeping audio aligned with video across pauses, gaps, and overlaps, and extending audio through the recording’s end.
  • Improved exported audio timing by removing encoder priming samples, helping audio start at the correct point.
  • Added recognition for .m4a audio files.

• Improvements

  • Screen recordings now maintain the last visible frame when the screen is idle.

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Reviewing files that changed from the base of the PR and between 76fadf5 and 7836058.

📒 Files selected for processing (2)

• electron/native/AudioTimelineTrack.test.ts
• electron/native/ScreenCaptureKitRecorder.swift

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📝 Walkthrough

Walkthrough

The macOS recorder maps video and audio to a shared timeline and writes aligned audio tracks through recording finalization. The media type map includes .m4a. The exporter uses stream and chunk timestamps to skip decoded audio priming frames.

Changes

macOS capture timeline

Layer / File(s)	Summary
Shared clock and audio timeline tracks electron/native/ScreenCaptureKitRecorder.swift, electron/native/AudioTimelineTrack.test.ts	The recorder adds a shared RecordingClock and AudioTimelineTrack. The track aligns audio buffers, inserts silence for gaps, trims overlaps, and writes audio output. The integration test covers timestamp scenarios and encoded output.
Capture timing and audio routing electron/native/ScreenCaptureKitRecorder.swift, electron/native/ScreenCaptureKitRecorder.test.ts	The recorder creates audio timeline tracks, routes audio callbacks on a dedicated queue, and uses the shared clock for capture timing. Tests cover pause and resume timing, idle-frame extension, and audio handling.
Recording end and sidecar finalization electron/native/ScreenCaptureKitRecorder.swift, electron/native/ScreenCaptureKitRecorder.test.ts, electron/mediaTypes.ts	Finalization limits audio to the stop time, drains callbacks, and pads tracks through the recording end. The media type map includes .m4a as audio/mp4. Tests cover audio buffering and padding.

Exporter priming-frame handling

Layer / File(s)	Summary
Count and skip decoded priming frames src/lib/exporter/audioProcessorShared.ts, src/lib/exporter/audioMediaProcessor.ts, src/lib/exporter/audioProcessorShared.test.ts	countPrimingFrames derives a frame count from stream and chunk timestamps. The decoder skips those frames, caps the count to available frames, and returns null if none remain. Tests cover priming metadata and unknown metadata.

Priority: ⬆️ High

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant CaptureStream
  participant RecordingClock
  participant AudioTimelineTrack
  participant AudioWriter
  CaptureStream->>RecordingClock: Map sample timestamp to recording timeline
  RecordingClock->>AudioTimelineTrack: Provide timeline timestamp
  AudioTimelineTrack->>AudioWriter: Write aligned audio buffers

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Suggested reviewers: webadderall

Merge Risk: ⚪ Minimal · up to 78360

The change adds shared-timeline audio alignment for macOS recordings and priming-frame trimming on export. No unresolved merge-blocking risk is evident from the supplied review evidence.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 78360

Capture permissions and recording destinations remain unchanged. The main identified risk is that prolonged encoder backpressure can accumulate queued audio without a limit, potentially exhausting memory and losing the recording.

Retained concerns

• Medium · reliability · inferred: The new inline-audio queue retains PCM without a byte or duration limit while AVAssetWriter remains writing but cannot accept audio. Sustained backpressure can therefore exhaust the native helper's memory before MP4 finalization, turning an encoder slowdown into loss of the recording. The base instead skipped nonready inline writes. Terminal-writer checks clear queued data, but the five-second drain deadline applies only after capture stops.

Security review details

Security Blast Radius

• inferred — The identified buffering failure is scoped to the local native capture session and its recording artifacts. The traced path does not grant captured audio or screen content authority to choose file destinations or obtain additional capture permissions.

Trust Boundaries and Controls

• observed — The helper retains screen-capture and microphone permission preflight. Inline writes require an accepting writer state, and draining additionally checks writer readiness. These controls constrain authority and terminal-writer misuse, but do not impose a resource budget on a stalled writer.

Resilience and Maintainability Implications

• observed — Explicit stop and parent-channel EOF reach native finalization. The stream-error delegate only logs the interruption in both the base and head, while Electron's interruption handling waits for process closure. Thus automatic stream-error cleanup is not guaranteed by these paths; the logging-only delegate predates this PR.

Hardening Proposals

• proposed — Give inline-audio buffering a byte or duration budget and an explicit recoverable outcome when the writer stalls, preserving independently written sidecar audio rather than allowing memory pressure to terminate the recording.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 32 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preventing dropped and late audio in macOS recordings.
Description check	✅ Passed	The description explains the problem, motivation, changes, test results, and reviewer testing steps. It includes screenshots and related issues. The Type of Change and Checklist sections from the temp…
Linked Issues check	✅ Passed	Issue #1016 requires .m4a sidecars to pass the media content-type allowlist as audio/mp4, without widening the path allowlist. electron/mediaTypes.ts adds this mapping. The reviewed changes do n…
Out of Scope Changes check	✅ Passed	The recorder, audio timeline, and exporter changes address the PR’s stated audio-loss, timing, and export-sync objectives. The tests cover these changes. The .m4a media mapping also implements issue…

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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/ScreenCaptureKitRecorder.swift:
- Around line 261-265: Update the inline-audio handling in write(_:) so it does
not enqueue sample buffers when inlineWriter is missing or has failed, been
cancelled, or completed; clear pendingInline in those states to release queued
buffers. Also bound pendingInline growth during silence filling or tail padding,
discarding excess buffers and warning when the cap is exceeded.

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• Configuration used: Repository: webadderallorg/Recordly/.coderabbit.yaml
• Review profile: ASSERTIVE
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📥 Commits

Reviewing files that changed from the base of the PR and between f24ce5c and 76fadf5.

📒 Files selected for processing (7)

• electron/mediaTypes.ts
• electron/native/AudioTimelineTrack.test.ts
• electron/native/ScreenCaptureKitRecorder.swift
• electron/native/ScreenCaptureKitRecorder.test.ts
• src/lib/exporter/audioMediaProcessor.ts
• src/lib/exporter/audioProcessorShared.test.ts
• src/lib/exporter/audioProcessorShared.ts

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