Fullscreen has some problems when you try to shift the scale/size.
Viewing post in Defeat the Demon lord - Demo comments
I'm looking into the issue right now. What I can tell you so far is that this is a very specific problem with the web version on itch.io. I exported the game to web to make it easier for everyone to try it out, but this issue doesn't happen on the desktop version. I’ll upload an update as soon as possible.