Woah, you weren't kidding about switching away from XMonad first. That is the most broken focus switching I've ever seen -- and that's after I figured out why I couldn't click anything in the window.
Ah, it seems like the pointer coordinates are offset from what Steam expects. If I move windows close to the center of the screen, or if I can find resize the window from an offset edge, it works again. With gnome-shell, fwiw.