Fixes #14959

Summary

Fix auto-scroll behavior when AI assistant streams responses in the web UI. Previously, the viewport would remain at the sent message position and users had to manually click a "new message" badge to see streaming responses.

Problem

When sending a message in the web UI:

1. User sends a message
2. Viewport stays at the sent message position
3. Assistant reply streams below the visible area
4. A notification badge appears, requiring manual click
5. User must click the badge to see the response

This creates a tedious conversation experience where users can't see responses in real-time.

Changes

• Reset chat scroll state before sending: Call resetChatScroll() before sending to ensure viewport readiness
• Force scroll after message send: Add force=true to scheduleChatScroll() to position viewport correctly
• Detect streaming start: Check when chatStream changes from null to a string value in handleUpdated()
• Auto-scroll during streaming: Trigger auto-scroll when streaming begins to ensure smooth scroll-following

Technical Details

• Modified ui/src/ui/app-chat.ts: Reset scroll state and force scroll on message send
• Modified ui/src/ui/app-lifecycle.ts: Detect streaming start and enable auto-scroll

Test Results

• ✅ All scroll tests pass (13/13 in app-scroll.test.ts)
• ✅ Full UI test suite: 192/197 tests passed (5 pre-existing failures unrelated to this change)
• ✅ Build successful

AI-Assisted Development

This PR was developed with assistance from Claude Opus 4.6. The code has been reviewed and tested to ensure correctness.

Co-Authored-By: Claude Opus 4.6 [email protected]

Greptile Overview

Greptile Summary

This PR adjusts the web UI’s chat auto-scroll behavior so the viewport follows assistant streaming responses.

Changes are primarily in the UI state/lifecycle layer:

• ui/src/ui/app-chat.ts resets chat scroll state before sending and forces a scroll scheduling after send.
• ui/src/ui/app-lifecycle.ts adds detection for when chatStream transitions from null/undefined to a string, and uses that to force an initial scroll when streaming begins.

Overall direction looks consistent with existing scheduleChatScroll() semantics (force only applies before the first auto-scroll), but there is a concrete type mismatch in the lifecycle host definition that should be fixed before merge.

Confidence Score: 4/5

• Mostly safe to merge once the lifecycle host type mismatch is corrected
• Behavioral changes are small and leverage existing scroll scheduling semantics, but LifecycleHost.chatStream is incorrectly typed as non-nullable while the rest of the UI sets it to null; this can mislead future edits and undermine the new streaming-start logic.
• ui/src/ui/app-lifecycle.ts

_{Last reviewed commit: 36a47a3}

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