The live thinking body pins to the newest tokens and keeps max-h-40
after the turn settles so the transcript doesn't jump. overflow-hidden
let that pin work in JS but clipped the rest of the thought. overflow-auto
makes the cap a real scroller; overscroll-contain keeps the wheel from
chaining into the transcript.
Supersedes #73757.
Co-authored-by: Dan Latimer <latdani@gmail.com>
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