Graphics are good. Frame rate is good. Resolution is a little high to play inside of a window? Maybe have a full screen button in the main menu? My only complaint was it was tough--- it would be fun if the first level was like--- fewer enemies, and less chances of taking damage!
You might have a really nice big monitor; so when you play it in the window the window prolly fits nicely in your window. But some people are playing 1080p, or even 720p monitors.. so the game is bigger than the window it's in. If that makes sense? It was able to play it, just had to put it on my bigger monitor! :D