Round-2 gate-cert proved the round-1 'snapshot.top + delta' + nearest-to-current
arbiter is mathematically unwinnable: a row's rect offset is scroll-relative
(rect.top - containerRect.top = rowContentPos - scrollTop), so a staged absolute
target only equals the true content-hold when scrollTop == snapshot.top — and in
that exact case the arbiter measures |raw - cur| = 0 and picks raw, writing the
stale value (the reported up-jump, unfixed).
Rework to the app's own realign idiom (mirrors _restoreMessageViewportAnchor and
_compensateScrollForMeasurementDelta): when a measurable anchor row exists, write
el.scrollTop += (currentOffset - capturedOffset) from the LIVE scrollTop — a no-op
when already aligned, a heal when not, no arbiter. Restore the per-tier lookup
guard (key -> sessionIdx, never the rawIdx degradation that maps to a different
message after a virtualization re-window). Genuinely-gone anchor: mirror the
topPad-delta idiom the snapshot already carries (topPadBefore), else keep raw.
Gate-cert tests now use scroll-dependent rect mocks (rect.top = rowContentPos -
scrollTop) so certified hold numbers are physically realizable; mutation-checked
(revert to raw -> the hold tests fail; drop topPad-delta -> that test fails).
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).
Frank Song