Fix DM reactions popup drifting off-screen on scroll (#14020)

## 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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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Ray Jacobson <raymondjacobson@users.noreply.github.com>

Author: 3 people

packages/harmony/src/components/popup/Popup.tsx

@@ -325,12 +325,18 @@ export const PopupInternal = forwardRef<
     (scrollParent: Element, initialScrollPosition: number) => {
       const scrollTop = scrollParent.scrollTop
       if (wrapperRef.current) {
-        wrapperRef.current.style.top = `${
+        const nextTop =
           originalTopPosition.current - scrollTop + initialScrollPosition
-        }px`
+        const adjustedTop = disableAutoFlip
+          ? nextTop
+          : Math.min(
+              Math.max(0, nextTop),
+              Math.max(0, window.innerHeight - wrapperRef.current.offsetHeight)
+            )
+        wrapperRef.current.style.top = `${adjustedTop}px`
       }
     },
-    [wrapperRef, originalTopPosition]
+    [disableAutoFlip, wrapperRef, originalTopPosition]
   )
 
   // Set up scroll listeners