isMiddleDown
.isMiddleDown()
Returns true if the middle mouse button is current pressed down, otherwise false.
Be aware that even today, not all users have a middle mouse button. So if you chose to use it, you should also provide an alternative. This will help to ensure everyone can play your game.
.isMiddleDown() { |x, y| ... }
Optionally, you can attach a block which will be executed if the middle button is currently down. The X and Y location of the mouse is passed in for you to use.
