Comment 9 for bug 801582

Ok, it most likely _is_ FocusIn / FocusOut and also most likely is grab related.

Here are the FocusIn and FocusOut events happening on a switch gnome-terminal -> gcalctool:

focusOut 119537694 (gnome-terminal), mode 1, detail 1
focusIn 422 (), mode 1, detail 2
focusIn 119537694 (gnome-terminal), mode 1, detail 5
---> switcher plugin initiated
focusOut 119537694 (gnome-terminal), mode 1, detail 5
focusOut 422 (), mode 1, detail 2
focusIn 146800729 ((null)), mode 1, detail 0
moveInputFocusToWindow 136314883 (galculator)
focusOut 146800729 ((null)), mode 2, detail 3
focusIn 119537694 (gnome-terminal), mode 2, detail 4
focusOut 119537694 (gnome-terminal), mode 0, detail 4
focusIn 136314883 (galculator), mode 0, detail 3
focusOut 136314883 (galculator), mode 0, detail 2

The first three are the keyboard grab becoming active due to Alt+Tab being pressed. The next three are the pointer grab becoming active through switcher. The last 5 happen after releasing Alt+Tab and thus releasing both grabs and moving the focus.
The printed data are the window, mode and detail members of the XFocusChangeEvent struct.

I don't have the time to do a deeper analysis at this moment (will do as soon as I have some time), but if you have some X/Gdk expert at hand, that'd be helpful ;)