Sorry for any confusion. Please let me clarify a little.

The thing I was getting at: is that front corner / edge of the top of the wall considered part of the top of the wall or part of the face of the wall? If it is part of the top of the wall wouldn't a fly ball hitting there be a home run? If it is part of the face of the wall wouldn't a fly ball hitting there be a ground rule double if it then went out of play or playable if it came back onto the field?

BTW I wasn't thinking of it resting on top of the wall.