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Two causes, both reproduced on a clone of a real long thinking-partner chat
(replies and thinking rows several screens tall, GFM tables).
1. The window lowered in e42771970 (initial 2 / batch 2 / window 5) holds only
two or three tall rows, so a fling outruns the mounted range and the viewport
crosses unmounted rows: 65 blank frames over a 4,152-frame traversal. Keep the
small initial render that bounds cold-open measurement, but refill a window of
10 in batches of 10: 0 blank frames over 4,347, p95 25 ms.
2. Scroll-to-bottom from deep history animated across every row in between,
which no finite window can mount in time: 9 blank frames (~150 ms) per jump.
Animate only when the current offset is within the bottom-side overscan,
otherwise jump instantly. Near-bottom jumps still animate. Web already does
not animate this jump.
Cold open on the same chat is unchanged (first content 569 -> 568 ms); the
deeper window costs first-scroll acknowledgement (2.58 -> 2.94 s in a debug
build).