Two halves of one pass; TODO-FIXES.md lists every finding.
The audit fixed, among ~90 items: MP4 export dead-ending and locking the UI
behind a stuck "Downloading..." button; height keyframes storing the width;
shadow defaults and keyframes storing undefined because fabric's get() is not
a path getter; every letter animation writing to the last letter; keyframe
times drifting on each drag because data-time was layer-relative on expanded
rows; O(n^2 log n) playback, now indexed once per frame; and a save() that
rebuilt the record canvas on every edit, now debounced.
Exports also lost audio-layer sound outright: MediaRecorder records only the
first audio track, so every source now mixes through one AudioContext into one
destination.
The new half is src/js/render.js, an offline renderer. Real-time capture was
the root cause of dropped frames on heavy scenes and smeared video layers: a
bare currentTime assignment is async, so drawing straight after it captures the
previous frame. Each frame is now seeked, awaited on 'seeked', drawn, and
pushed through a VideoEncoder; audio is mixed in one OfflineAudioContext pass
and encoded to Opus. webm-writer2.js gained a second Opus track and lost three
bugs, including a MAX_CLUSTER_DURATION_MSEC of ~58 days that overflowed the
signed 16-bit block timecode past ~32s.
Real-time capture remains the fallback: the hand-rolled muxer's output is
decoded in a <video> element before being handed over, and any failure returns
null so record() falls back transparently.
test/webm-muxer.test.js parses the muxer output with an EBML reader in plain
Node. It caught the fixed-256-byte header overflow that inspection missed.
Not verified in a browser: node --check passes on every script and the muxer
test passes, but a running Chrome held the Playwright profile lock, so the
manual pass listed at the end of TODO-FIXES.md is still outstanding.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
