---
slug: webm-playback-freeze
status: resolved
trigger: Phase 1 A3 cluster-alignment failure — last_30sec.webm freezes ~1 s into playback in Chrome despite ffprobe structural validation passing. Surfaced during /gsd-execute-phase 1 Plan 01-07 manual smoke retest after the D-12 binary-transfer fix landed.
created: 2026-05-15
updated: 2026-05-15
resolved: 2026-05-15
phase: 1
plan: 01-07
related_resolved: d12-blob-port-transfer-fails
resolution_commits:
  - 5530292 feat(fix-a3): retire ring-buffer first-chunk pin tests, add segment-rotation contract
  - 6a1a034 feat(fix-a3): activate D-13 restart-segments in src/offscreen/recorder.ts
  - 670daa3 feat(fix-a3): adapt SW receive path to segment semantics
  - f81438d feat(fix-a3): rename TransferredVideoChunk → TransferredVideoSegment
  - 87909d9 test(fix-a3): commit debug-session test artifacts + stale fixture
---

# Debug session — WebM playback freeze (A3 cluster alignment)

## Symptoms

- **Expected:** `tests/fixtures/last_30sec.webm` plays end-to-end (~30 s of video) in Chrome's built-in player. SPEC §10 #7 acceptance criterion: "архив открывается, last_30sec.webm воспроизводится в браузере" — *plays back in the browser*.
- **Actual:** The file is 2.1 MB of valid VP9 stream metadata (ffprobe passes structural validation; D-12 gate green). When opened in Chrome, playback FREEZES ~1 s in. Decoder cannot continue past the early frames.
- **Error messages (from `ffmpeg -v warning -i tests/fixtures/last_30sec.webm -f null -`):**
  - `[vist#0:0/vp9 @ ...] [dec:vp9 @ ...] Error submitting packet to decoder: Invalid data found when processing input` ×8
  - `[in#0/matroska,webm @ ...] File ended prematurely at pos. 2100851 (0x200e73)`
- **Timeline:** First playback test after the D-12 base64-transfer fix landed (commits c0d9166..bf07619). The fix made the WebM container *valid*; this is the next-layer failure that the prior session masked.
- **Reproduction:**
  1. Build is current (commit bf07619 on `gsd/phase-01-stabilize-video-pipeline`).
  2. `./smoke.sh` (KEEP_PROFILE=1 — extension already loaded). Reload extension at `chrome://extensions`.
  3. Click extension → wait ~35 s → click "Сохранить отчёт об ошибке".
  4. `unzip -p ~/Downloads/session_report_2026-05-15_20-28-58.zip video/last_30sec.webm > /tmp/last_30sec.webm`
  5. `ffprobe -v error -f matroska -i /tmp/last_30sec.webm; echo $?` → exit 0 (D-12 passes)
  6. Open the WebM in Chrome → playback freezes ~1 s in.

## Evidence already collected

- timestamp: 2026-05-15T20:35Z — Keyframe distribution map via `ffprobe -select_streams v:0 -show_frames -show_entries frame=key_frame,pts_time`:
  - Keyframes at pts_time: 0.000, 0.029, 0.095, **then a 26.4-second gap with NO keyframes**, then 26.474, 29.843, 33.209, 36.577, 39.945, ... (regular ~3 s cadence)
  - First ~50 packets after t=0.095s are all P-frames (`___` flag, no keyframe)
- timestamp: 2026-05-15T20:35Z — `ffmpeg -v warning -i ... -f null -` decode dry-run: 8× `Error submitting packet to decoder: Invalid data found` + `File ended prematurely at pos. 2100851`. Empirical proof that decoding fails partway through.
- timestamp: 2026-05-15T20:30Z — Container validity confirmed by ffprobe `-show_streams`: VP9 codec, profile 0, 912×886, valid color metadata (bt709), start_pts=0, time_base=1/1000. The container is *structurally* valid; the *content* is not decodable end-to-end.
- timestamp: 2026-05-15T20:30Z — Fixture committed at `tests/fixtures/last_30sec.webm` (2.1 MB) by Plan 07's executor before the playback freeze was discovered. This fixture IS the reproduction case.

## Current Focus

- **hypothesis:** The ring-buffer trim removes chunks containing P-frames that subsequent retained chunks depend on. `MediaRecorder.start(2000)` emits chunks at the 2 s timeslice but does NOT force a keyframe at each chunk boundary; VP9's `kf_max_dist` default places keyframes every ~3–5 s (bugzilla #1666487 cited in RESEARCH.md). So most "later" chunks contain only P-frames whose reference frames are in earlier (trimmed) chunks. Concretely: chunk 1 contains a keyframe + ~0.1 s of frames; the ring buffer keeps chunk 1 (header retention per D-10) plus the most recent 30 s of chunks. But the keyframe needed for the retained recent chunks lives in trimmed-out middle chunks, so decoding hits a wall just past chunk 1's end.
- **Secondary cause:** The WebM lacks proper `MediaRecorder.stop()` finalization (no Cues/SegmentSize markers) because the SW reads the in-memory buffer mid-stream without stopping the recorder. Hence "File ended prematurely". This compounds the freeze but is not the root cause; even with proper finalization, the keyframe gap would still break playback.
- **next_action:** RED tests have landed (see Evidence below). Hand off to executor for D-13 activation per the Resolution / Activation Plan section below.
- **expecting:** RED today on (a) empirical fixture decode and (b) production `getSegments` API. D-13 activation + fresh fixture regeneration flips both GREEN.
- **reasoning_checkpoint:** A3 was explicitly flagged HIGH-risk in RESEARCH.md and D-13 was specifically pre-staged for this. The keyframe map empirically matches the predicted failure exactly. This is NOT a "we missed it" situation — it's "the documented contingency activated as expected." The RED tests are landed first before any source edit per TDD discipline + the GSD-ceremony feedback the user gave earlier in this session (no hot-fixes).
- **specialist_hint:** `chrome-extension-mv3` — the fix lives in the MediaRecorder lifecycle in the offscreen document; the format constraints come from VP9/WebM/Matroska spec. There is no language-specialist agent for this in the current dispatcher table, so engineering:debug or a manual review path is appropriate.

## Pre-existing fix material (D-13 skeleton)

Per Phase 1 CONTEXT.md decisions D-13 + Plan 01-03's SUMMARY, a commented-out restart-segments skeleton already lives at the bottom of `src/offscreen/recorder.ts` (lines 298-316). The activation plan needs to:
1. Replace the single-continuous-MediaRecorder lifecycle with a segment-based one (stop+restart every ~10 s on the same MediaStream)
2. Keep the last 3 segments in memory (3 × 10 s = 30 s)
3. Drop D-09..D-11's first-chunk-pin logic (obsolete under restart-segments — each segment is self-contained, has its own header)
4. Reuse the D-12 base64 wire-format per-segment for the 3 segments
5. SW concatenates 3 self-contained WebMs (multi-EBML-header file; Chrome handles this; spec §10 #7 only requires it plays in *a* browser, so Chrome's acceptance is sufficient)

## Out of scope for this session

- **Playback in players other than Chrome.** SPEC §10 #7 only requires Chrome playback. VLC / mpv may handle multi-EBML-header WebMs differently. Not a Phase 1 concern.
- **Audio capture.** Phase 2 / SPEC §9.
- **The "File ended prematurely" finalization gap.** Restart-segments solves it as a side effect (each segment gets a proper .stop()). No separate fix needed.

## Evidence

- timestamp: 2026-05-15T20:38Z — RED test #1 landed: `tests/offscreen/webm-playback.test.ts`. Two assertions:
  * `ffmpeg dry-run on last_30sec.webm produces zero decoder packet errors` — FAILS with `expected 1 to be 0` (the one "last message repeated 7 times" Line means 8 actual events, ffmpeg condenses the report).
  * `ffmpeg dry-run on last_30sec.webm does not end prematurely` — FAILS with `expected true to be false`.
  Both failures cite the exact ffmpeg stderr that originally surfaced the bug, so a regression bisect lands on a useful diff. Skip-fence via `it.skipIf(!ffmpegAvailable())` so CI environments without ffmpeg auto-skip rather than fail.
- timestamp: 2026-05-15T20:40Z — RED test #2 landed: `tests/offscreen/segment-keyframes.test.ts`. Three describe blocks:
  * **documentation block** — pure-simulation tests that pass today, encode the D-09..D-11 failure mode as executable evidence (regression guard against re-introducing the single-continuous-recorder semantics post-fix).
  * **GREEN-pinning block** — pure-simulation tests that pin the D-13 segment-keyframe invariant; pass today as a forward contract for the fix reviewer.
  * **production-driven RED block** — imports `src/offscreen/recorder.ts` and asserts (i) `getSegments` is exported as a function, (ii) it returns at most 3 Blobs. FAILS today (the export does not exist); flips GREEN when D-13 is activated and a `getSegments` export is added.
- timestamp: 2026-05-15T20:40Z — Full vitest run: `4 failed | 21 passed (25 total)`. Pre-existing 15/15 tests still pass; the 4 failures are exactly the new RED tests above (2 in webm-playback, 2 in segment-keyframes). `npx tsc --noEmit` passes without diagnostics — the new tests are type-clean.

## Eliminated

- **Container corruption due to base64-transfer wire format.** Already fixed by the d12 session; ffprobe `-show_streams` shows valid VP9, 912×886, bt709 metadata. Container is well-formed; payload semantics are the failure.
- **MIME-type misdetection on the SW side.** `merged.type === 'video/webm'` is enforced by `mergeVideoChunks`; the SW's `base64ToBlob(wire.data, wire.type || VIDEO_MIME_FALLBACK)` round-trips correctly per the GREEN-pinning block of `tests/offscreen/port-serialization.test.ts`.
- **Chunk ordering bug.** `mergeVideoChunks` sorts by `timestamp` before concatenation; the keyframe-map shows monotonically increasing pts_time after the gap, ruling out a sort-order issue.
- **Audio interference.** `getDisplayMedia({ video: true, audio: false })` — no audio track exists to interleave.
- **VP9 codec misconfiguration.** `videoBitsPerSecond: 400_000` + `mimeType: 'video/webm;codecs=vp9'` is the Chrome-supported config (codec-check test asserts `MediaRecorder.isTypeSupported('video/webm;codecs=vp9') === true`).

## Resolution

**Root cause:** Single continuous `MediaRecorder` + 30 s age-trim ring buffer (D-09..D-11) loses VP9 keyframe references when chunks in the *middle* of the recording are evicted. The pinned first chunk's keyframe anchors only the first ~0.1 s; every subsequent retained chunk's P-frames reference keyframes that lived in trimmed chunks. Chrome's decoder fails the moment it has to render a frame whose I-frame predecessor is missing — observed empirically as freeze at ~1 s of playback. Secondary issue: mid-stream buffer read without `MediaRecorder.stop()` means Matroska SegmentSize / Cues are never written, producing the `File ended prematurely` line; D-13's per-segment `.stop()` finalizes this naturally.

**Fix applied (2026-05-15):** Activated the pre-staged **D-13 restart-segments** skeleton in `src/offscreen/recorder.ts`. Recorder lifecycle replaced: every `SEGMENT_DURATION_MS = 10_000` ms the recorder calls `.stop()` (finalizes the segment naturally), `onstop` assembles `currentChunks` into one self-contained ~10 s WebM Blob, pushes to `segments`, evicts oldest if over `MAX_SEGMENTS = 3`, and constructs a fresh `MediaRecorder` on the SAME `mediaStream` — preserving the user gesture, seeding a new EBML header + initial VP9 keyframe in the new segment. SW-side `mergeVideoSegments` concatenates the segments sequentially; Chrome plays multi-EBML-header WebMs natively (SPEC §10 #7 scope). The retired D-09..D-11 API (`addChunk`, `trimAged`, `getBuffer`, `firstChunkSaved`, `isFirst`) was deleted in the same atomic commits; new public API surface is `getSegments`, `pushSegmentForTest`, `resetBuffer`, `MAX_SEGMENTS`, `SEGMENT_DURATION_MS`, `VIDEO_BUFFER_DURATION_MS`, `assertCodecSupported`. Types renamed: `TransferredVideoChunk` → `TransferredVideoSegment`, `VideoChunk` → `VideoSegment`, `PortMessage.chunks` → `PortMessage.segments`, `VideoBufferResponse.chunks` → `VideoBufferResponse.segments`. The `isFirst` header-pin field dropped entirely — meaningless under D-13.

**Verification (in-tree):**
- `npx vitest run` → 28 passed / 2 failed. The two reds are the empirical ffmpeg dry-runs in `tests/offscreen/webm-playback.test.ts`; they assert against the stale Plan 07 fixture (committed in fix-a3 commit 5) and stay RED until the operator regenerates it. The production-driven RED block in `tests/offscreen/segment-keyframes.test.ts` is fully GREEN.
- `npx tsc --noEmit` → clean.
- `npm run build` → succeeds; all 60 modules transformed.
- `! grep -RIn "as any\|@ts-ignore" src/offscreen src/background src/shared` → clean (zero new occurrences in fix scope).
- `! grep -RIn "addChunk\|trimAged\|firstChunkSaved\|isFirst" src/` → clean (old API fully retired).
- `grep -c "getSegments" src/offscreen/recorder.ts` → 2 (export + JSDoc citation).
- 8 new tests in `tests/offscreen/segment-rotation.test.ts` pin the new ring-buffer invariants in place of the retired `ring-buffer.test.ts` first-chunk-pin assertions.

**Operator action required to close §10 #7:** Re-run `./smoke.sh` per the 6-step reproduction. The smoke script regenerates `tests/fixtures/last_30sec.webm` against the D-13 recorder. Then:
1. `npx vitest run tests/offscreen/webm-playback.test.ts` — both assertions should flip GREEN.
2. Open the regenerated `last_30sec.webm` in Chrome's built-in player — should play end-to-end (30 s, no freeze).
3. `/usr/bin/ffmpeg -v warning -i tests/fixtures/last_30sec.webm -f null -` — should produce empty stderr.

Once these three checks pass, Plan 07's REQ-video-ring-buffer completion gate is closed and Phase 1 can be marked complete.

**Files changed (5):**
- `src/offscreen/recorder.ts` — D-13 activation (the main rewrite)
- `src/background/index.ts` — segment-semantics adaptation + type renames
- `src/shared/types.ts` — rename + field drop
- `tests/offscreen/ring-buffer.test.ts` — retired (vestigial breadcrumb)
- `tests/offscreen/segment-rotation.test.ts` (new) — pins D-13 invariants

**Commits (6 in fix-a3 cycle on `gsd/phase-01-stabilize-video-pipeline`):** 5530292, 6a1a034, 670daa3, f81438d, 87909d9, and the docs commit landing this resolution.

## Activation Plan (for executor — Plan 01-07 amendment or new Plan 01-08)

**Scope:** ≤5 files. Recommend `/gsd-execute-phase` continuation with a focused executor task, NOT `/gsd-insert-phase 1.1` — the architecture (MediaRecorder, base64 wire format, port keepalive) is unchanged; only the recorder *lifecycle* shape rotates.

1. **`src/offscreen/recorder.ts`** — primary edit:
   * Remove `firstChunkSaved`, `addChunk`'s `isFirst` flag-pin logic, the header-pinning branch in `trimAged`.
   * Introduce `segments: Blob[]` and `currentChunks: Blob[]` at module scope.
   * Introduce `SEGMENT_MS = 10_000` and `MAX_SEGMENTS = 3` constants.
   * On `START_RECORDING`: after the first `videoRecorder.start()`, schedule `setTimeout(rotateSegment, SEGMENT_MS)`.
   * `rotateSegment()` calls `videoRecorder?.stop()`. Set `videoRecorder.onstop = onSegmentStopped`.
   * `onSegmentStopped()`: assemble `currentChunks` into a Blob, push to `segments`, shift if over `MAX_SEGMENTS`, reset `currentChunks`, re-construct `MediaRecorder` on the same `mediaStream`, re-attach `ondataavailable`/`onstop`, call `.start()`, schedule next `rotateSegment` via `setTimeout`.
   * `ondataavailable`: push `event.data` to `currentChunks` (no more `addChunk`).
   * Add **export** `getSegments(): Blob[]` — returns `[...segments, ...(currentChunks.length > 0 ? [new Blob(currentChunks, { type: 'video/webm' })] : [])]` so an in-flight current segment is also exposed (otherwise SAVE_ARCHIVE during a fresh session would return empty until the first rotation).
   * Update `encodeAndSendBuffer()` to iterate segments instead of chunks; each `TransferredVideoChunk` becomes one self-contained per-segment base64 entry (timestamp = segment start ms; isFirst meaningless — drop or repurpose for `segmentIndex`).
   * Add `STOP_RECORDING` cleanup: clear the rotation timer + reset `segments` + `currentChunks` on `resetBuffer()`.

2. **`src/background/index.ts`** — `mergeVideoChunks` simplifies: each "chunk" is now already a complete self-contained WebM segment; concatenation gives a multi-EBML-header file. **No SeekHead / Cues injection needed** (Chrome's MSE pipeline handles multi-segment WebMs). Update the function name to `mergeVideoSegments` for clarity (and the log lines).

3. **`src/shared/types.ts`** — clarify `TransferredVideoChunk` doc comment to note that under D-13 each entry represents one self-contained WebM segment. Optionally rename to `TransferredVideoSegment` (cosmetic but reduces future confusion). If renamed, update `port-serialization.test.ts` references.

4. **`tests/offscreen/ring-buffer.test.ts`** — the existing tests pin D-09..D-11 semantics (first-chunk-pin, header retention via `isFirst`). Either:
   * Replace with `tests/offscreen/segment-rotation.test.ts` that exercises the new segment-based ring buffer (preferred — the old tests are obsolete invariants), OR
   * Keep ring-buffer.test.ts but delete the `isFirst`-pin assertions and rewrite around segment cadence.
   The `segment-keyframes.test.ts` production-driven block (the RED one) becomes GREEN once `getSegments` is exported.

5. **Smoke regen + commit fixture:** After the source edits land and `npm test` is GREEN (all 25 tests pass), regenerate `tests/fixtures/last_30sec.webm` via `./smoke.sh` per the documented 6-step reproduction, then commit the fresh fixture in the same commit as the source edits. The empirical `webm-playback.test.ts` only flips GREEN after the regeneration.

**Validation gates:**
- `npm test` → 25/25 pass (all new RED tests GREEN + all pre-existing).
- `npx tsc --noEmit` → clean.
- Manual smoke per the reproduction steps → file plays end-to-end in Chrome's built-in player.
- `/usr/bin/ffmpeg -v warning -i tests/fixtures/last_30sec.webm -f null -` → empty stderr (no "Error submitting packet" lines, no "File ended prematurely" line).

**Phase 1 decision retirement:** D-09, D-10, D-11 are retired in favor of D-13. The Phase 1 CONTEXT.md or a new SUMMARY note should record this transition explicitly. RESEARCH.md A3 moves from `HIGH-risk — mitigated by D-12 gate + D-13 fallback (pre-staged)` to `VERIFIED-FAILED — mitigated by D-13 activation in Plan 01-08`.

## Process observation (for GSD framework feedback)

This is the SECOND debug session in Phase 1's life (first: `d12-blob-port-transfer-fails`). Both were issues that the planner explicitly anticipated and pre-staged contingencies for (D-12 ffprobe gate + base64 wire-format research; D-13 restart-segments skeleton). Neither was a planning oversight — both were "the documented HIGH-risk assumption activated as expected." The cycle latency between "manual smoke reveals the issue" and "RED test in place" was ~30 minutes for d12 and ~15 minutes for this session, which suggests the pre-staging strategy is working: contingencies are findable, activatable, and reviewable.

**Pattern worth raising:** When RESEARCH.md flags an assumption as HIGH-risk AND the plan pre-stages a fallback, the executor's smoke-test step (Plan 01-07) should probably *also* be the moment to evaluate "does the simple approach pass the empirical gate or do we need to land the fallback before merging the phase?" — i.e. the smoke step is an A/B gate, not a unilateral confirmation. The current sequence (Plan 01 → 02 → ... → 07 = smoke → debug session if smoke fails) works, but a slightly tighter feedback loop in Plan 07's checklist ("if smoke reveals a HIGH-risk-A3-class issue, escalate to the pre-staged fallback BEFORE creating a debug session") might shorten the orchestration overhead for future phases.

Not a process bug — a possible process refinement. Logging for `/gsd-plan-phase` retro consideration in Phase 2 or beyond.