Describe the bug
The Panel Selector modal in the grading interface is draggable.
Two issues were observed:

1. When dragged outside the viewport it vanishes and becomes
   inaccessible. Reopening the modal causes it to appear at
   the same off-screen position.
2. When dragging and releasing the modal anywhere, the modal
   vanishes immediately even without going off screen.

Refreshing the page is the only way to reset it back to center.
This was observed in all grading roles including TA, grader, and instructor.

Expected behavior
The modal should be constrained within the viewport boundaries
and should not vanish when dragged and released.

To Reproduce

1. Go to the grading interface as any grading role
2. Click the panel mode button
3. Drag the Panel Selector modal to any position and release
4. The modal vanishes immediately on release
5. Click the panel mode button again to reopen
6. Modal appears at the last dragged position
7. If dragged outside the viewport, modal becomes completely
   inaccessible and reopening still shows it off screen
8. Refreshing the page is the only way to reset the position

Also reproducible by simply dragging and releasing the modal
anywhere. It vanishes immediately on release.

Configuration

• OS: macOS
• Browser: Chrome, Edge, Safari

Media
Video demonstrating the bug: https://imgur.com/a/cqUDS7I

Additional context
No additional context.