GIMP on OSX comes with a little program to enable X11 Focus Follows Mouse on OSX. This means that all you have to do to focus an X11 Window is to move your mouse over it. Sounds great so I ran the program, turns out it isn’t so great and is in fact REALLY annoying. Does the GIMP ship with a program to turn X11 Focus Follows Mouse off again? Hell no, that would just be far too considerate of them!

However, fear not, some googling lead me to the answer, to turn it off just run the following from your command prompt:

defaults write com.apple.x11 wm_ffm -bool false

No prizes for guessing how to turn it back on or for how to set it for other apps!