Simon c60f5e01dc Re-anchor a moving reel on where it is, not what it was playing ...

A viewport change re-anchors the reel on the video that is playing,
resolved by id rather than by a scroll position the change has already
invalidated. That is what keeps a rotation on the same video.

Mid-swipe it names the wrong one. setActive runs from onScroll's next
frame, so a resize landing before that frame still reads the video being
swiped *away from* -- and realignToActive writes its position straight to
scrollTop, hauling the reel back to it. A phone slides its chrome on every
scroll, so the resize lands mid-swipe on nearly every swipe: the reel went
where the thumb put it, far enough to show the top of the next video at
the bottom of the screen, and was then written back a beat later.

The input guard in onScroll could not catch this. There is no scroll event
to guard -- the write happens inside the resize.

So while the reader is demonstrably driving, take the reel's own position
as the authority and re-anchor on the step it is nearest at the new
height. It still lands on a snap point, so nothing is left straddling two
screens, but the swipe stands. Only a step's worth of disagreement counts
as theirs: a scroller just re-parented into or out of the
picture-in-picture window reads 0, and following that would land them on
the first video instead of their own.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

2026-10-27 22:26:39 +00:00

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