Proton 6.10 GE

You need a little tweak to initiliaze full screen: 1- Open the game normally and enable full screen mode, the game might crash or a black screen might appear. 2- If the screen is black close the game. 3- Re-open the game, a black screen will appear. 4- Click super key and then come back to game. Now there shouldn't be any issues with fullscreen. (You need to this step everytime you open the game.)