Boxes

No menu (or at least no documented way to open one).
Through trial and error, I discovered that 'Q' is an undo key. However, there are a limited number of undos.
I hoped that 'R' would reset the puzzle, but this was not the case.
So to reset the puzzle, I have to restart the game...

Q to undo and hold Q to reset the game. Pretty neat little puzzle game.