This fixes F12 not working if you clicked on a window's title bar to get back window focus, and you have a player control such as Ring Toss mapped to the left mouse button (which is a game default I believe). Turns out the game basically doesn't realise you let go of said button apparently!
Note: The workaround for 2.1.20 is to either move the mouse inside the window and click again, or press the keyboard button for the control (if you have one set). Obviously this depends on what your controls actually are, use your common sense if necessary please. =)