fix(desktop): stop chat scroll jumping by disabling native scroll anchoring#37866
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…horing The thread renders virtualized turns in natural document flow with padding spacers, and @tanstack/react-virtual already adjusts scrollTop itself when an off-screen turn is measured and its real height differs from the 220px estimate. With the browser default `overflow-anchor: auto`, native scroll anchoring corrects that SAME size delta too, so the two double-correct and the view lurches — most visibly with Windows mouse wheels, whose coarse notches mount/measure several under-estimated turns per tick (Mac trackpads scroll ~1-3px/frame, keeping it sub-perceptual). Set `overflow-anchor: none` on the thread viewport so only the virtualizer compensates. Also adds `diag-scroll-reset.mjs`, a CDP wheel-up repro that A/B tests the anchor behavior at runtime to confirm the fix.
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Jun 5, 2026
…roll-anchor fix(desktop): stop chat scroll jumping by disabling native scroll anchoring
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Summary
Fixes the reported bug where the chat scroll position "resets randomly" while scrolling up to read — reproducible on Windows with a mouse wheel, invisible on a Mac trackpad.
Root cause: the thread (
apps/desktop/src/components/assistant-ui/thread-virtualizer.tsx) renders virtualized turns in natural document flow (padding spacers, not transforms, so theposition: stickyhuman bubble works).@tanstack/react-virtualuses a flatestimateSize: () => 220and then dynamically measures each turn, adjustingscrollTopitself to keep content stable. The scroller had the browser defaultoverflow-anchor: auto(it was set nowhere in the app), so native scroll anchoring corrected the same size delta a second time → double-correction → the view lurches.A Mac trackpad scrolls ~1–3px/frame so the over-correction is sub-perceptual. A Windows mouse wheel delivers ~120px notches that mount/measure several under-estimated turns per tick, so the double-correction becomes a visible "reset" you have to fight.
The custom sticky-bottom anchor and the incremental external-store runtime were ruled out:
runStartonly fires on a realisRunningfalse→true transition, and the upward-scroll disarm logic is sound.Changes
apps/desktop/src/styles.css— setoverflow-anchor: noneon[data-slot='aui_thread-viewport']so only the virtualizer compensates (one-line fix, with a comment to prevent regression).apps/desktop/scripts/diag-scroll-reset.mjs— CDP diagnostic that drives synthetic mouse-wheel-up scrolling and A/B testsoverflow-anchor: autovsnoneat runtime, tracking a mid-thread turn's on-screen position. Matches the existingscripts/diag-jump.mjs/measure-jump.mjstooling.Test plan
node apps/desktop/scripts/diag-scroll-reset.mjsagainst the app (launched with--remote-debugging-port=9222); confirm "reverse jumps" / "big lurches" / "native scroll moves" drop sharply in theoverflow-anchor: nonerun vsauto.scrollTopwrites, unaffected by disabling native anchoring).🤖 Generated with Cursor