Two linked causes of the mobile scroll jump-back, both confirmed by the
maintainer against origin/master in #5637.
Primary (CSS): the @media (pointer: coarse) block applied a flat
`content-visibility: auto; contain-intrinsic-size: auto 1px` to every .msg-row.
Off-screen assistant rows can be multi-thousand px (tool-result / long answers),
so a 1px reservation collapses their off-screen height; scrollHeight lurches down
by tens of thousands of px on a long transcript, the browser force-clamps
scrollTop, and the viewport is thrown to the top. A flat intrinsic-size cannot
both reserve tall rows and keep scrollHeight stable for iOS flick momentum (why
1px was chosen), so content-visibility is scoped to the short, size-predictable
user rows; assistant rows keep real rendered height. Mirrors the existing
#liveAssistantTurn opt-out pattern.
Secondary (JS): _restoreMessageScrollSnapshotSameFrame fell back to an absolute
snapshot.top when the semantic anchor restore failed. During streaming the live
activity-scene refresh fires this every tick; for a reader scrolled up into
history (unpinned), snapping to a stale absolute top nudges the viewport backward
by an amount that grows with scrollHeight. Now: hold position (no scrollTop write)
for the unpinned/anchor-failed case and let the browser's own scroll anchoring
keep the reader put. Pinned/near-bottom readers keep the tail-relative restore.
Adds tests/test_issue5637_mobile_scroll_clamp_jumpback.py (mutation-checked:
reverting either fix fails the matching assertion).
f657014aa3
