Very nice short game, I wish there were more levels with more mechanics as each level progresses. And also, tip of advice, the reason the window is like that when you fullscreen is not because of a bug, but because many game engines do that. I personally use gamemaker and to fix that problem, I made the width and height of the game to the browser's size. You could look up a tutorial how to fix this, I'm sure there is one!
Besides that, this game was a nice little game :)