fix(desktop): prevent scroll rubber-banding from virtualizer measurement adjustments

[Colinchiu007]

Problem

对话内容总是自动跳到上下一段距离，不是固定在我阅读的位置。

Root Cause: The conversation uses @tanstack/react-virtual virtual list. After messages render, async operations (Shiki syntax highlighting with 120ms delay, image loading) change item heights. The virtualizer adjusts scroll offset to "preserve viewport position" — but this creates a feedback loop: measure → adjust → re-measure → re-adjust, producing visible rubber-banding / content jumping.

The existing AT_BOTTOM_THRESHOLD guard (added previously) only covered the "parked at bottom" case. When the user is reading mid-conversation, there was no protection.

Fix

1. New constant MEASUREMENT_NOISE_THRESHOLD = 50 — any virtualizer scroll adjustment ≤ 50px is treated as measurement noise, not user intent.

2. Updated scrollToFn with two guard cases:

   • Case 1 (existing, refactored): User at bottom → skip adjustments that would scroll up
   • Case 2 (new): User mid-conversation → skip adjustments ≤ 50px threshold

The browser's native CSS overflow-anchor: auto handles real content growth/shrink without feedback loops.

Testing

This is a UX fix for intermittent scroll jitter. Best verified by:

1. Load a long conversation with code blocks (Shiki highlighting) and images
2. Scroll to middle of conversation
3. Verify viewport stays stable as messages below/above finish rendering

Related

Closes the "对话内容跳动" issue.

[teknium1]

Thanks for identifying the scroll-jitter failure mode. This is now superseded by current main; this is an automated hermes-sweeper review.

• 76b93869d8edddab860e225b11ef9e04df33ba63 (fix(desktop): rebuild thread autoscroll on use-stick-to-bottom) deleted apps/desktop/src/components/assistant-ui/thread-virtualizer.tsx, so the PR's scrollToFn target is no longer present.
• apps/desktop/src/components/assistant-ui/thread/list.tsx:109 makes use-stick-to-bottom the single scrollTop owner, avoiding virtualizer measurement corrections competing with another scroll controller.
• apps/desktop/src/styles.css:1016 disables native anchoring while following and restores overflow-anchor: auto once the user has scrolled away from the bottom, preserving the mid-conversation reading position behavior this PR addresses.

Closing as implemented on main via 76b93869d8edddab860e225b11ef9e04df33ba63 (included in v2026.6.19).