Copperbox revision 2990.
I've added another control / motion experiment - Pachinko.
In a nutshell, it seems that holding down the left or right arrows for constant motion is not going to work. So I need a new control strategy that uses singular key presses. Pachinko seems like a good model - start the flea off with maximum velocity and let its acceleration very slowly decay to zero. Jumping is still controlled by a key press (so there is some deviation from Pachinko), but left or right direction is determined by the initial motion and collisions rather than user action.