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).
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