"the video never presented frame 1; it offered 2" was this function waiting
for a frame it had not asked for a second time. The walk discarded a wrong
frame and re-armed the callback WITHOUT seeking again, and nothing further is
ever presented to a paused element that has not been asked to move — so one
wrong answer starved until the timeout and reported it as the browser refusing.
Underneath that, the wrong answer was not wrong. A container can carry an edit
list, and `currentTime` then counts from the start of the edited presentation
while a frame's `mediaTime` counts from the start of the media. The two differ
by a constant, so the frame at currentTime 0 can honestly report a mediaTime
two frames in. Seeking cannot correct for it in the positive direction: source
frame 0 would have to be found before the start of the video, every attempt
clamps at zero, and the walk offers frame 2 forever.
So the constant is measured once and subtracted. `calibrate!` takes whatever
the browser calls the first frame it shows and makes that the origin, which is
the honest definition anyway — it is what a viewer sees at time zero, and the
audio clock this take plays against starts in the same place.
Calibration also leaves the element on frame 0, so the walk starts holding it.
`frame!` returns immediately for a frame already on screen rather than seeking
to where it already is, which presents nothing and would hang.
Residual error still re-seeks, corrected by exactly the measured miss, and
still fails loudly with every frame that was offered and where it was asked
from, so a next failure is diagnosable in one shot rather than four.
The proxy also drops B-frames now. That removes the edit list at the source
rather than only coping with it, and makes decode order presentation order
should this ever be fed to WebCodecs. 14% larger, and extraction refuses a
proxy whose timeline is shifted so it cannot come back silently. Honest note:
this was my first diagnosis and it did NOT reproduce the failure — both
proxies walk correctly here on hardware decode — so it is hardening, not the
fix.
Verified by forcing the fault: a harness that offsets the reported timeline by
-2, -1, 0, +1, +2 and +5 frames failed on every positive offset before and
recovers on all six now, first attempt. Real app in headed Chrome with Metal
hardware decode: 280/280. 41 backend and 234 frontend tests green.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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