When the muxer patched a byte range inside an already-written chunk (the
mdat header, for example), the merge trimmed the right side of the old
segment but left its full blob on the left, duplicating megabytes and
shifting every box offset — the MP4 still reported its duration but had
no usable video track (0x0 in the browser, "moov atom not found" in
ffprobe).
The collector/assembler now lives in its own module and was verified in
Node with a real transmux of J-81: the assembled file matches the source
(h264 1280x720, 500 frames, AAC, 20.84 s) byte for byte in structure.
JakeBreath
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