Stopping a game

Steam's Stop button works: confirm "Exit game?" and the game and everything it started end within a few seconds, even while the game is still loading. Steam's button goes back to Play.
In Superposer: Stop Game… on the running game in the sidebar, in the Game menu, in the menu bar extra, or in the in-game overlay. Superposer first closes the game's window like the close button, so it can save; if it's still running after 20 seconds it is ended, and unsaved progress is lost.
Quitting from the game's own menu is always best.