That is a very helpful insight. The fact you just mentioned suggests that the game should quit if you press the "ESC" button on your keyboard and only the UI interface appears to be inactive.
I will need some information about your special case to fix both of the problems:
- What is your Linux distribution version?
- What is your monitor screen size in pixels?
- Do you have any programs running while playing the game? (You might not notice them because of the "Running in the background" feature of Unity)
- Have you had this particular problem with other Unity games? If so, which parts of your screen were inaccessible?