Fix DM reactions popup drifting off-screen on scroll

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Summary

• Fixes a UI positioning bug where the DM reactions popup could drift upward/off-screen after opening.
• Updates Harmony popup scroll tracking to clamp computed top position to viewport bounds when auto-flip/clamping is enabled.

Root Cause

• Popup initially clamps its position when opened, but the scroll listener later recomputed top without clamping.
• In scrollable chat containers this allowed the popup to keep moving upward and leave the viewport.

Changes

• packages/harmony/src/components/popup/Popup.tsx
  • In watchScroll, compute nextTop and clamp to [0, window.innerHeight - popupHeight] unless disableAutoFlip is enabled.
  • Add disableAutoFlip to callback deps.

Validation

• Manual code-path validation of popup positioning logic.
• Attempted targeted lint command, but local workspace tools were unavailable (eslint: not found) in this environment.

  

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