After Alt-Tabbing, when switching back to the game, it couldn't fullscreen. By that I mean it looked like the game wanted to fullscreen, but something was making it windowed again, so it looped the animation fullscreen->windowed->fullscreen .... several times a second, and since it was changing all the time, the screen was black. The game was still fine, I could interact with menus and move the character even if I couldn't see what I was doing.

Warning! linux native based save files aren't compatible with windows native/proton/GE save files. I tried switching to Proton to fix an alt-tabbing issue, but my saves crash the game. I was already thorugh half of a chapter so I grit my teeth and continued on native without alt tabbing.