AS3 MouseEvent, ROLL_OVER, MOUSE_OVER
Essential ActionScript 3.0 p.536

MouseEvent.ROLL_OVER + MouseEvent.ROLL_OUT does not trigger an "out" event when moving from a parent to its descendant. Unlike MOUSE_OVER + MOUSE_OUT, the event doesn't bubble.

addEventListener(MouseEvent.ROLL_OVER, overHandler, true);
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