Yeah, I'm not sure how to fix that. I'm using Unity 2018.2.14f, which is pretty recent. As I mentioned in the development builds, there's a workaround:
"Linux builds might start with bugged graphics. To solve this, go to ~/.config/unity3d/Flaming Torch games/Fairtravel Battle/, edit the prefs file, and change the fullscreen line to have a value of 0, and remove the two lines that give a width and height of 1."
Sorry for the trouble, though. Whenever I figure out how to properly fix this, I'll do so. It's a critical bug on Linux.