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tlongwell-blockandDawn 6b04aa1e49 fix(desktop): make upscroll compensation engine-order-independent (WebKit)

The T2 realization-compensation writer in useAnchoredScroll.ts was
near-perfect on Chromium and near-ineffective on WebKit (39 felt lurches
up to 204px per 30-swipe momentum pass). Root cause: the reading-anchor
baseline was snapshotted in the scroll handler, on the assumption that
scroll fires before ResizeObserver in the same frame. That holds on
Chromium; WebKit dispatches scroll asynchronously off its scrolling
thread and coalesces it, so under momentum the RO consumed a stale
snapshot and the drift measurement folded the user's own wheel delta
into the correction — the writer fought the wheel.

Make the path independent of scroll-event timing:

- Baseline the reading anchor per-rAF (single writer, module rAF loop),
not per-scroll-event. rAF runs before layout/RO on every engine, so
the baseline is the freshest pre-reflow read cross-engine.
- Compute the correction scroll-invariantly from the anchor row's
document position (scrollTop + topOffset), which changes only when
content above reflows — isolating the layout shift from user scroll.
Extracted as computeAnchorCorrection(), unit-tested for sign and
invariance.
- Write an absolute scrollTop target from this frame's own late reads,
not scrollBy against a possibly-stale offset (blunts the write-race).
- Staleness is a first-class SKIP: if scroll moved more than
COMPENSATION_SCROLL_SKIP_PX since baseline, momentum is in flight and
the reads may straddle a compositor commit — skip and let the next
quiet frame catch it. Under-correcting is invisible; fighting the
wheel is the lurch.
- Derive at-bottom vs mid-history from synchronous geometry
(isAtBottomNow) in both the rAF sampler and the RO callback, never
from the scroll-maintained anchorRef, so the branch and the baseline
can't disagree under momentum.
- Pick the reading anchor a safe margin below the fold
(READING_ANCHOR_SAFE_MARGIN_PX) so it never sits inside the
realization band it's meant to measure against.

Co-authored-by: Dawn (sprout agent) <c6237ef84fa537c78dcee78efd2d4e59f728859c7f194da42ac51ededfa0be05@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>

2026-07-08 18:13:49 -04:00