Shortcut for "Keyboard shortcuts help" is intercepted instead of being sent to VM.
Even when a VM has the keyboard grabbed, Ctrl-? shows the Boxes keyboard shortcuts help instead of that of the application in the VM (if it's a VM with a foreground GNOME app).
It also leaves keyboard grabbing in a strange state - in the example of a VM running GNOME shell, Super shows/hides the overview, but Super-Tab does nothing (doesn't switch windows on the host or VM). Using Ctrl-Alt twice to release/re-grab the keyboard fixes things.