So I didn’t want to spend any cash on PhotoShop and installed The Gimp on my MacBook. Unfortunately, The Gimp runs in X11 and that gives one little but very annoying problem: everytime I change window (e.g.: from Tools to the picture window itself), I have to click twice to do the action I want: first to get focus on the window, and then to do the action I want to do.
defaults write org.x.X11 wm_click_through -bool true
Enter it in a terminal, and restart X11.