While minecraft is open in windowed mode and you select a map in single-player, while the map is loading you resize the window with the Maximize button (the one between the X and the _ TOP RIGHT.) the actual size of the window is maximized but the game remains on the default resolution stuck at bottom left of the window. You can fix it by doing the same thing again once. I can reproduce it with newly created maps as well.

This only works on that specific moment when the map loads. And i guess it is for slower PC's only because I can't reproduce it on my other PC that is twice as fast as the one i am using now.

Not something great but i thought i should report it anyway.