As per the title, you can't actually do anything when in full screen.
Solved
It seems to be working for me…
Though a normal problem is that HTML Iframes can’t get keyboard input until you first click into the frame itself, this is a little counterintuitive without a title screen/any notification, but prevents stuff like keylogging.
I’ll add a start menu in the next build to fix this.
Also, for anyone else, please leave bug reports in the respective topic. (derpygamer2142 is chill because this post is from before it was made.)