When a mouse button is pressed over a QWidget Qt grabs the mouse and directs the events back to the widget even as the mouse leaves the widget. If the mouse pointer is moved to another X11 screen than the one the widget is on and the button then released then the ButtonRelease event isn't properly processed. If you move the mouse pointer (now without holding a button) back over the widget it starts processing mouse-motion events as if the button was still pressed down.