MOUSE_DOWN, MOUSE_UP events work only if I set the style to my SkinnableContainer

Question

I am designing a physics container. Don't worry, my question is not  about the physics. Here is my code. Noticed that I have a  SetStyle statement there. I realized that if I don't have that line, the  MOUSE_DOWN, MOUSE_UP events won't work. If I do have that line, then  those events works fine. Anyone know the reason?
package components
     import flash.events.MouseEvent;
     import spark.components.SkinnableContainer;
     public class Test extends SkinnableContainer
          private var mouseDown:Boolean;
          public function Test()
               super();
               //setStyle("backgroundColor", "0xBBBBBB");
               addEventListener(MouseEvent.MOUSE_DOWN, mouseDownEventHandler);
               addEventListener(MouseEvent.MOUSE_UP, mouseUpEventHandler);
          private function mouseDownEventHandler(mouseEvent:MouseEvent):void {
               mouseDown = true;
          private function mouseUpEventHandler(mouseEvent:MouseEvent):void {
               mouseDown = false;

Answer 1

Mouse events only fire on "drawn pixels".  If there are no pixels drawn, the
mouse events fall through to the next thing in the background.