Oh, I think that's it. I tried running it in an Incognito tab and was able to reproduce the problem. It doesn't currently support running with local storage disabled.
I'll try investigating this over the weekend, but you should be able to play it if you run in a regular tab.
I'm not sure what would cause this, something is causing it to fail to sync the virtual file system, but I'm not sure what would do that. Possibly some sandboxing problem? You could try clearing offline site data and see if that helps.
I just checked it in both Firefox and Chrome and both are working fine for me. Do you have any additional log messages or anything that could give more information about the error?
Probably not much since there is already an SDL/OpenGL/POSIX backend which compiles in Cygwin with CMake and is the basis of the macOS port, but I don't have a Linux machine to develop on or test it on so I'm not sure what else is needed.
If you're interested in testing or contributing to a Linux version, please comment on issue #2 on the GitHub page.
Yes, you can turn on fullscreen mode by pressing Alt+Enter at any time or by going to Options -> Preferences -> Display and checking "Full screen mode"
Due to browser restrictions, if you do the second option, you'll need to press a key or click the game frame after closing the Preferences window to transition to fullscreen.
