fix(web): restore thread scroll position

[harshiempire]

What Changed

Switching away from a thread and back now returns the chat timeline to where you were reading, instead of jumping to the latest message. Positions are kept per client session in a small bounded in-memory cache keyed by thread. A thread you left at the bottom keeps following new content as before; only a thread you had scrolled up in is restored.

Scope decisions:

• Web and desktop only. Mobile has its own timeline and is consciously not covered.
• Nothing is persisted across restarts or synced between devices.
• Citation positioning still takes priority over a saved position.

Why

Glancing at another thread and losing your place in a long conversation is a constant small annoyance when driving several agents. Provenance: #6195 (transcript half; the open-file half is #9874).

The saved offset is written once per animation frame while scrolling and cleared when the reader reaches the end, so following live output costs nothing extra.

UI Changes

Video: scroll up in a thread, switch to another thread, switch back. (attached below)

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (motion change, video instead)
• I included a video for animation/interaction changes

Written with Claude Fable 5.1 in Claude Code.

🤖 Generated with Claude Code

Note

Restore per-thread scroll position in MessagesTimeline

• Adds timelineScrollState.ts module with a bounded LRU cache (200 entries) to store numeric scroll offsets per thread key.
• MessagesTimeline.tsx reads the saved offset on mount, disables end-follow during restoration, and saves or clears the offset on scroll and cleanup.
• ChatView.tsx starts routed threads in free-scrolling mode when a saved position exists.
• Adds delete method to lruCache.ts to support removing individual cached entries.
• Risk: ChatView timeline initialization switches from end-following to free-scrolling for threads with a saved non-end position.

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