In the user interface, the "buttons" (the 'main menu' button at the
top of the screen, and the balls at the bottom) are too sensitive: they
respond whenever the mouse is released while the pointer is over them.
Since a major part of the game consists of dragging the mouse around the
screen to control the camera, it's very easy to accidentally release the
mouse in the wrong place and have the menu pop up or (very annoyingly)
the camera recenter on another ball.

  The buttons should only respond to mouse-up events when a mouse-down
was previously received *on the button* (in which case the camera
shouldn't respond)

  Daniel