User reported: after pressing Enter on a long thread, the view jumps up
— the just-submitted message disappears below the fold. Confirmed via
apps/desktop/scripts/measure-jump.mjs:
before: distFromBottom 0 → 49.5px, sticks there permanently
after: distFromBottom 0 → ~0 (worst case 4px for one frame)
Root cause in useThreadScrollAnchor (thread-virtualizer.tsx):
1. The sticky-bottom logic disarmed on any scroll event where
`scrollTop < lastTopRef.current`. That check can't distinguish a
user scrolling up from a programmatic `pinToBottom` write that
the browser clamped short of bottom (because content also grew in
the same frame, so `scrollTop = scrollHeight` lands at
`scrollHeight - clientHeight` for the OLD scrollHeight, which is
now below the NEW scrollHeight). Result: sticky-bottom disarmed
permanently on the user's first submit.
2. There was no synchronous pin tied to React's commit phase. By the
time the ResizeObserver fired and re-pinned, the user had already
seen ~50ms of "message below the fold" — visually that reads as the
view jumping up.
Fix:
- `programmaticScrollPendingRef` counter tracks scroll events we
expect to be ours (one per `pinToBottom` write). The scroll handler
skips the disarm check when consuming a pending tick, keeps the
arm bit true, and re-pins synchronously if the browser clamped us
short of bottom. A depth cap (8) breaks runaway loops in
pathological streaming-burst layouts.
- `useLayoutEffect` on `groupCount` increase pins BEFORE the browser
paints, eliminating the visible ~50ms window between optimistic
user-message insert and the RO/scroll-event chain firing.
Verified on the long Cloud Shadows thread (7-8 turns, ~11k px tall):
all three repro runs now hold within 0–4 px of bottom across the
post-Enter transition. Submit latency unchanged (paint 77–107 ms),
streaming-typing latency unchanged.
Also adds three debug harnesses:
- measure-jump.mjs — sample thread scroll across Enter
- probe-thread.mjs — dump current thread / scroll state
- diag-jump.mjs — intercept scrollTop + RO + mutations across Enter
