Even though Fullscreen was activated, the taskbar sometimes covered the bottom of the screen, making some things difficult to see. Deactivating Fullscreen solved the problem, and it was actually borderless windowed afterwards

The first start of the game crashed instantly to Desktop, after selecting Proton 4.11 it started. But crashed later on while ingame

The lower left info box was empty. It should instead show unit information like entrenchment and action points. This made the game unplayable, as this info is vital. And even right-clicking and going for the units properties does not show all the info that are supposed to be in the info box.