After rearranging panes, the window becomes permanently unmovable.
Grab handles temporarily set `window.isMovable = false` on hover to prevent
window dragging from interfering with pane dragging.
Override `viewWillMove(toWindow:)` to catch when the view is being removed from
the window. This lifecycle method is called before the window reference
becomes nil, allowing us to restore `window.isMovable`.
