1. Drag the orange square over the blue square.
  2. Without releasing the drag, click the middle mouse button.
  3. If the platform's normal behaviour is to cancel a drag (eg. Windows and Unix+KDE), then the drag should be cancelled;
  4. If the platform's normal behaviour is not to cancel a drag (eg. Mac and Unix+Gnome), then the drag should not be cancelled;
  5. Fail in either case if an inappropriate middle click function begins (eg. paste-and-go).