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.
e67ab2e042