Thats not a bug, because the title and buttons stick to the corners of the game.
I cant make it do that, while only adhering to the boundaries of the box within (mutually exclusive), so to do that would make it awkwardly sit centered if you increased the size of the game at all.
Using Expand is supposed to make it fill the whole screen anyway.
Now, Im not the best CSS person on earth, so if someone thinks im an idiot and can make it do both things (follow the corners of the game but only until it reaches the holding box) im ready to apply it! Note that the expansion feature itself is a little bit of css voodoo, since reflowing anything with the frame causes the flash to refresh itself, which is a very very bad side effect, so it does it with margins and junk.