I’ve been working on papervision dragging and to detect when the mouse cursor leaves and goes off/outside the stage bounds in AS3. (Or the browser if the movie is set to scale to fill the window) one would use this listener:

mc.stage.addEventListener(Event.MOUSE_LEAVE, mouseLeaveHandler);

fucntion mouseLeaveHandler(e:Event):void{}

However this event doesn’t fire when the mouse is down while the mouse leave the stage.