I'm running the game on Ubuntu 25.10 with GE-Proton10-27. There's no exit option in the game itself, Alt-F4 doesn't work, clicking the X on the title bar doesn't work, and right-clicking the icon on the task bar and choosing "Quit" doesn't work. I've resorted to closing the game by clicking the stop icon in Heroic, which appears to forcefully terminate the process. This wouldn't be that big a deal, except that the game saves your key bindings on exit. Since there's no way to exit gracefully, the key bindings never get saved, so I have to configure them again every time I start the game.