fix: prevent scroll position jump when AI completes response while user reading

[Louis454545]

Describe Your Changes

• Fixed scroll position jumping issue when AI completes response while user is reading in the middle of the conversation
• Added tracking of user's intended scroll position during streaming
• Implemented logic to restore user's reading position when streaming completes instead of auto-scrolling to bottom
• Preserved existing auto-scroll behavior when user is genuinely at the bottom

Fixes Issues

• Closes bug: Scroll position jumps up when the text is completed and the scroll position not on bottom. #5939

Technical Implementation

• Added userIntendedPositionRef to track where user deliberately scrolls during streaming
• Added wasStreamingRef to detect streaming completion transitions
• Modified scroll handlers to record user's position when they scroll away from bottom during streaming
• Updated auto-scroll effect to restore intended position when streaming ends
• Used smooth scrolling for position restoration to provide better UX

Testing

The fix addresses the exact reproduction steps from the issue:

1. Ask AI a question that generates a long response
2. While AI is generating, scroll to middle of the response to read
3. When AI completes, scroll position now stays at user's reading position instead of jumping to top

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed

🤖 Generated with Claude Code

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Important

Fixes scroll position jump by tracking and restoring user's intended scroll position during AI response streaming in ThreadDetail.

• Behavior:
  • Prevents scroll position jump when AI completes response while user is reading.
  • Tracks user's intended scroll position during streaming and restores it when streaming ends.
  • Maintains auto-scroll behavior when user is at the bottom.
• Implementation:
  • Adds userIntendedPositionRef and wasStreamingRef in ThreadDetail to track user scroll position and streaming state.
  • Modifies scroll handlers to record user's position when scrolling away from bottom during streaming.
  • Updates auto-scroll effect to restore intended position with smooth scrolling when streaming ends.
• Testing:
  • Fix verified by reproducing issue steps: asking AI a question, scrolling to read, and ensuring position is maintained when AI completes.

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[ellipsis-dev]

Important

Looks good to me! 👍

Reviewed everything up to 9462aaf in 1 minute and 35 seconds. Click for details.

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1. web-app/src/routes/threads/$threadId.tsx:157

• Draft comment:
  The check to clear the intended scroll position (if streaming starts) is unreachable because wasStreamingRef is updated immediately before. Consider storing the previous streaming state (e.g. using a temp variable) before updating wasStreamingRef, then use that in the condition.
• Reason this comment was not posted:
  Comment looked like it was already resolved.

2. web-app/src/routes/threads/$threadId.tsx:143

• Draft comment:
  Using a fixed 100ms delay in the setTimeout for DOM updates might be fragile. Consider a more robust approach (e.g. requestAnimationFrame or a MutationObserver) to ensure the DOM is updated before scrolling.
• Reason this comment was not posted:
  Confidence changes required: 50% <= threshold 50% None

Workflow ID: wflow_G34SO29VJT4vxTnd

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[urmauur]

LGTM