## Summary
- virtualize MessageTimeline so large session histories stop rendering
the full timeline sidebar at once.
- keep the existing full render path in selection mode so xray/selection
behavior stays intact.
- route active-segment scrolling through the virtualizer so timeline
navigation still follows the selected message.
## Benefit
- prompt field was very laggy in cession with big history and timeline
had many bugs, this is fixed.
- the session with big history now load as fast as a new session .
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