Return true if the middle mouse button has been released on the current frame, and otherwise returns false.
A click differs from being down by only being recorded for one frame, rather then permanently, whilst the button is pressed.
If you do check for a middle click, ensure your game can be played without it. Even today, not all users have a mouse with a middle mouse button.
You can optionally attach a block onto 'isMiddleClick' which is executed if the middle mouse button is being clicked.