Before this change, the hitbox felt one-sided because the cursor didn't
consistently change into the drag-handle cursor.

The reason was that we didn't trigger a redraw on hover, so we'd only
change the cursor if we detected hover AND something else caused a
redraw.

Release Notes:

- Fixed the pane resize handler not consistently triggering on mouse
hover.

Co-authored-by: Antonio <antonio@zed.dev>