New items arrive on every streamed token, and _onChanged jumped to
maxScrollExtent unconditionally — so a reader who scrolled up was
dragged back to the tail continuously for the whole reply. The
_atBottom pin already existed for viewport resizes (T-297); apply it
to the new-item path too, re-checking after layout since the user can
scroll during the frame. Twin tests added beside the T-297 pair:
pinned view keeps following, scrolled-up view stays put.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
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