Thank you for checking the game out! Yeah, there's tons of things to fix, the health/hitbox issues being the main. Weapons too, Buffson is not going to be the same. It's underway but I will probably save everything for one big update; there's a lot of story elements I completely left out due to time.
As for the resolution thing, it might be a browser issue? I remember testing another Jam Godot game with Brave and Firefox and I noticed their fullscreens behaved differently. I'll test again when I update, but honestly Godot browser is just full of weird little issues.