[Obi]

private var _isOver:Boolean = true;
private var _coords:Point = new Point();
 
if((stage.mouseX < 0 || stage.mouseY < 0 || stage.mouseX > stage.stageWidth || stage.mouseY > stage.stageHeight) && _isOver) {
     _coords.x = stage.mouseX;
     _coords.y = stage.mouseY;
     _isOver = false;
     dispatchEvent(MyEvent.MOUSE_LEAVE);
}else if(!_isOver) {
     _isOver = true;
     dispatchEvent(MyEvent.MOUSE_ENTER);
}

Как-то так, наверное.