fix: chat auto-scrolls to top after thinking completes

[xoknight]

Summary

• Fix streaming auto-scroll effect that was a no-op: scrollContainerRef has no fixed height and never overflows, so el.scrollTop = el.scrollHeight did nothing. The real scrollable ancestor is the parent div in chat-area.tsx.
• Replace with bottomRef.scrollIntoView({ behavior: 'instant', block: 'end' }) which correctly targets the nearest scrollable ancestor.
• Change activeBubbleId scroll from block: 'nearest' to block: 'end' — the active bubble is always the latest message, and nearest could leave the viewport mid-list after thinking transitions.

Root Cause

In chat-session.tsx, three useEffect hooks manage auto-scroll:

1. Effect 1 (msgCount dep): bottomRef.scrollIntoView — works correctly
2. Effect 2 (session.messages dep): scrollContainerRef.scrollTop = scrollHeight — broken because the element has no height constraint and never overflows
3. Effect 3 (activeBubbleId dep): activeBubbleRef.scrollIntoView({ block: 'nearest' }) — works but nearest can scroll to mid-list

When thinking completes, onAgentEnd removes empty messages and onAgentStart adds new ones. If React batches these, msgCount stays the same → Effect 1 doesn't fire. Effect 2 should compensate but is a no-op → scroll position is lost during re-layout → chat jumps to top.

Test plan

• Start a conversation, let AI think, verify scroll stays at bottom when response arrives
• During text streaming, verify chat stays pinned to bottom
• Scroll up manually during streaming, verify auto-scroll doesn't force back down (isAtBottomRef guard)

🤖 Generated with Claude Code