So the default max resolution that has no cutoff for me is 1920 x 1080, my max resolution. even if I drop it down to 1680 x 1050 it cuts off. Same with next resolution down, 1600 x 900. I am running the Brave Browser on Zorin Linux. The grey still stays if I hit escape but what I have noticed is that if I click on the grey then it loads in fullscreen fine, even at lower resolution. I also tested it on ungoogled chromium and its the exact same.
Well ideally I would like feedback on my game carved in tablets of solid gold and encrusted with diamonds, hand delved by a beautiful nymphomaniac.
But if that is a little bit much just give me your feedback here. Whatever jumps out at you. After trying out the first map I would take a peek at map 4 and 12 if I were you. Those are the most visually impressive.