XFCE: active panel buttons when mouse is on screen edge

"It appears I've figured it out on my own... It's a ""feature"" of compiz. The solution is explained here:
http://lists.compiz-fusiondotorg/pipermail/dev/2009-January/001146.html

Compiz uses small, 2 pixels wide windows to detect edge actions.
Unfortunately, it's not possible to pass the enter/leave/motion events
to the window beneath with the X infrastructure as it stands :-/
You probably want to disable the top and/or bottom edges in the wall
plugin (under ""Key bindings"" -> ""Edge flipping"").

I don't depend on that edge flipping feature very much, so this fixes it for me."