I don't understand what it's supposed to do, it just spawned three windows, one plain yellow and the two others with 3 stripes and an increasing counter. Is that a bug or did I miss something? (I tested with up-to-date firefox and chromium, same thing)
If the windows are not moving, then it is a bug (although, check that you haven't clicked pause on the game). Otherwise, they should be moving and recreating the game pong. The plain yellow is the ball, the striped are the paddles for the characters (controlled by A&Z for the left one and UP&DOWN for the right one). Tick one of the boxes as non human to play against a computer.
Oh, the windows are supposed to be the game! Ok, then it's not a bug (per se) but I'm using a tiling wm, it's preventing the windows from moving. Thanks for the explanation.
