When switching to a session with long context (especially in Electron
desktop when changing servers), the chat viewport could render blank
until the user scrolled. Two interacting issues caused this:
1. historyVirtualRows memo never recomputed after the first render
because historyVirtualizer (from useVirtualizer's useState) is a
stable reference. Frozen range meant items rendered at the top
while paddingBottom filled the visible viewport after scrolling.
2. pendingInitialRestoreRef replay ran in useEffect (after paint),
showing a frame at scrollTop:0 with the stale virtualizer range.
Fixed by: adding a virtualVersion counter driven by useVirtualizer's
onChange to bust the memo; switching the replay to useLayoutEffect
so scroll position is set before the browser paints.
Tom
db3558f906