Alt+Tab or changing Plasma workspace will crash the game, every time. To avoid this, I installed the Windows version of Steam and the game through Lutris, with the Card Hunter install script for Lutris. That makes it possible to Alt+Tab and change workspace as much as I want. Weird workaround, I know, but since the SteamPlay version crashes when I Alt+Tab I recommend Lutris if anyone else get the same issue.

Right-clicking freezes mouse input, weirdly enough. You can right-click on some things OR use spacebar, so since mouse input stops working if I press right-click I recommend using the spacebar. I still get this issue when I use the Windows version of Card Hunter through Lutris, but I can Alt+Tab or change workspace and then go back again and mouse input will then be working again. Since the SteamPlay version crashes when using Alt+Tab this workaround can't be used with it, so I recommend installing Card Hunter through Lutris if this happens to you as well.

Crashes when using Alt+Tab or changing Plasma workspace. Which means if I right-click and mouse input stops working I have no workaround and can't continue the game. Using the Lutris version to avoid it.