Hey, so I've exported a project of mine to zyhrllos-projects.000webhostapp.com/magneticball
I've tested it on 2 browsers, currently: Chrome and Firefox. It has a fullscreen button at the bottom right.
It works like a charm on Chrome! Exactly as intended. However, no dice with Firefox. It might have something to do with the whole "no fullscreen without user input" rule, but it only toggles when you push the button. Any help?