I haven't done this yet, but will be working on it shortly.
It looks like the MOUSE_UP event has both a currentTarget and
target properties. I'm not sure I get the difference at the moment,
but that is where I would start.
Event.currentTarget is always equal to Event.target for non
display objects but have a look at this example of display objects
containing other display objects
so typically currentTarget ->what registered the listener
target -> event target object