@Wkot, to prevent the Who actor from sinking below the bottom of the screen, its y-coordinate must be kept below a point in the world that is half the actor's image height less than the world height. That expression gives that y-coordinate of the line that the center of the actor is not allowed to go beyond in the downward direction.

@Roshan123, move repetition and stalemate are both drawn games (so is draw by agreement, which is obviously not incorporated in this game). As for you 2nd comment, insufficient material was not implemented.

"Irregular Shape Collision Demo" is a bit long and could be misleading, here. However, the documentation does show what is needed to bounce off any smooth surface (curved or not). Complications do occur at bounce points that do not have a normal to the surface and finding the bounce point itself (to find the correct normal to use) could be tricky with irregular surfaces