I just purchased and installed MewnBase via the itch.io app on my iMac (Mojave 10.14) and when I launch it the game shows up in the lower left quarter of the screen. Oddly the mouse tracking is still full screen. I have to guess and use the button focus on the menus to help click around. We've tried full screen mode and high-res mode in all combinations and it is the same result. Noteworthy the game loading screens and loader all run full screen with out any problems.
I wonder perhaps if this is related to the retina screen.