That's kinda weird, the only time this happens to me is when the game loses focus. In that case, a simple left button click in the window area fixes it.
Yep, this should work with any game downloaded as a .zip, but not all games have the .x86_64 file extension for the executable (for non-Unity games, the executable file name is usually the same as the name of the game)
1) download the zip file
2) move it in a directory of your choice
3) extract it ('unzip Karlson_Linux.zip' in the terminal)
4) make the .x86_64 file executable ('chmod +x Karlson_linux.x86_64')
5) run the executable with './Karlson_linux.x86_64'
