Strange bug, easily reproducible. Autohide gnome panel, then click mintmenu and pick any program. Click on it. Gnome panel then does not disappear UNTIL you click somewhere on the panel. Then it disappears. When I go into synaptic and removed mintmenu, the problem disappeared. Of course I tried many things before that, like removing compiz and so on, but nothing worked until I removed mintmenu. Then with the normal gnome menu, autohide works fine. A shame because I LIKED the mintmenu! This is isadora version (Mint 9).