"Easy fix would be add overflow: hidden to the body tag. "

I noticed if I zoom in, then they show up, too. But just adding overflow hidden would make me also not see the whole screen anymore - so in your case, the automatic screensize adjustment seems not to work.

That's why you set the canvas size to scale with the screen. Can be done with CSS or JS. Overflow works, I do that with all my webgl fullscreen projects.

Yeah well, me too. And this project seems to do so, too. At least for me on different screensizes. So it likely is a bug why it does not work for you.