I found that the MouseEvent.getX()/getY() could sometimes return negative value (-1 but very close to 0) for a mouse_moved event when I move the mouse to the top left hand corner. But isn't 0 the leftest that the mouse could move on a Component? Further mouse move should simply means the event generated to the background container, never to this component.
posted 6 years ago
My mistake. Looks like it is not mouse_moved event. Rather it is mouse_exited.