Switches the ConversationPage chat list from a full O(N) render to
windowed rendering via react-virtuoso. On long histories only the
visible rows (plus a 400px overscan buffer) live in the DOM, ending the
scroll jank and layout thrashing that hit conversations with >500
messages.
Preserved behaviors:
- Newest message visible on open via initialTopMostItemIndex.
- Auto-scroll on send via a pending-count-based effect (the old
setStickToBottom + useLayoutEffect pattern doesn't apply now that
Virtuoso owns the scroll element).
- Realtime auto-follow only when scrolled to bottom (followOutput).
- 'New messages while away' counter + 'jump to newest' pill via
atBottomStateChange.
- Pinned-message / reply / search jumps via virtuosoRef.scrollToIndex;
expands displayCount on the fly if the target is outside the
rendered slice. Flash highlight unchanged.
- Load-older infinite scroll via Virtuoso startReached (replaces the
IntersectionObserver-on-sentinel pattern).
- Per-conversation position memory now keys on row index instead of
pixel scrollTop (the latter isn't meaningful under virtualization).
Also wires the PinnedMessagesPanel onJump callback (previously a TODO
that just closed the panel) into jumpToMessage, since virtualization
made the smooth-scroll-from-pinned UX easy to deliver as a side
benefit.
byGalax
d3b708636f