iOS Safari portrait resolves its dynamic toolbar height after first paint; when the
toolbar collapses the #messages scroller grows (clientHeight up), firing a scroll
event with decreased scrollTop that was misread as an upward user scroll -> falsely
set _messageUserUnpinned, and the ResizeObserver settle then self-cancelled, leaving
the reader stranded at the TOP of a freshly-opened conversation (landscape unaffected
since it doesn't resize late). Sibling of #4701.
Fix (touch-safe, no-op on desktop/landscape where clientHeight is stable):
- Track _lastMessageClientHeight; gate movedUp on !grew so a container-height
INCREASE can't be read as an upward scroll. Reset on session switch.
- For explicit (open) settles, also observe the scroller itself so a late portrait
toolbar collapse re-anchors the bottom.
Independent of #4818 (overflow-anchor paint-gap) — different mechanism, coexists.
Diagnosis credit: investigation. Co-authored-by: reinocheong <reporter>
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