fix(tui): compensate scroll on message prune - #41247

Issue for this PR

Closes #41243

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

While the agent is producing output, a user can scroll up to read earlier messages. Once a session exceeds 100 messages, every new message prunes the oldest one off the top of the message store (packages/tui/src/context/sync.tsx). The scrollbox holds scrollTop fixed while scrolled up, so removing content above the reading position drifts the viewport down the transcript on every new message.

This PR compensates the scroll position when the oldest messages are pruned, keeping the reading position stable while the user is scrolled up:

• packages/tui/src/util/scroll.ts: compensatePruneScrollTop returns a scrollTop moved up by the height of content that disappeared above the first surviving message, computed from the layout captured before the prune.
• packages/tui/src/routes/session/index.tsx: runs it in a createComputed watching the message list, so it executes before the list re-renders while the pre-prune child layout is still available. Sticky-bottom following is untouched — when the user is at the bottom, compensation is skipped and the scrollbox re-sticks as usual.
• AssistantMessage now wraps its parts in <box id={message.id}> so every message (user and assistant) renders an identifiable box. Previously only user messages carried an id, so pruned assistant messages could not be measured.

How did you verify your code works?

Added packages/tui/test/util/scroll-prune.test.tsx: unit tests for compensatePruneScrollTop plus a render test (testRender from @opentui/solid) mirroring the session wiring. It fills 100 alternating user/assistant messages, scrolls up 10 lines, then simulates the sync.tsx prune on each new message and asserts the message pinned at the top of the viewport stays at the same viewport line — without the fix it drifts from y = 2 to y = -22 across 6 prunes.

Verified bun typecheck (packages/tui) passes and the full packages/tui suite passes (200 tests, 8 snapshots unchanged).

Screenshots / recordings

N/A (terminal UI; no recording).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR