Probably because the face detection library is blocking the event loop
so the menu (and other things) are not able to process their paint events.
http://wiki.wxpython.org/LongRunningTasks
http://wiki.wxpython.org/Non-Blocking_Gui
http://wiki.wxpython.org/CallAfter
--
Robin Dunn
Software Craftsman
http://wxPython.org