- useVirtualHistory: track last-seen ScrollBox metrics in a ref inside
the post-layout effect and bump ver when sticky/top/vp change — the
subscribe-based rearm was sufficient for fresh clicks but not for the
"hydrated mid-commit, measured empty, then metrics settle" path where
nothing re-triggered the hook until the next unrelated keystroke
- useSessionLifecycle: resume scrollToBottom from queueMicrotask to
setTimeout(..., 0) so the fresh transcript has a full task turn to
commit + measure before we try to land at the newest content
useVirtualHistory set up its useSyncExternalStore subscription during
the first render, when scrollRef.current was still null (the ScrollBox
ref attaches during commit, after render). Its useCallback for
subscribe had a stable scrollRef identity as its only dep, so it never
re-subscribed once the ref actually attached — the hook stayed stuck
with vp=0, top=0, no scroll subscription. Small sessions fit entirely
in cold-start so you didn't notice; big /resume sessions got sliced to
the last 40 items with a huge topSpacer and the viewport sat on empty
space until some unrelated state change (e.g. a keystroke) re-rendered
and finally read a real vp.
- flip a hasScrollRef flag in useLayoutEffect once the ref attaches and
add it to the subscribe useCallback deps so useSyncExternalStore
rearms with a real subscription
- on resume, scrollToBottom() after history hydrates so the ScrollBox
lands at the newest messages instead of scrollTop=0 (stickyScroll
doesn't auto-engage on the initial empty→full dump)
