code: function(event) {
                  var button = event.button;
                  if (!event.which && button !== undefined)
                      return (button & 1 ? 1 : (button & 2 ? 3 : (button & 4 ? 2 : 0)));
                  else
                      return event.which;
              }
docs: Gets the clicked mouse button of the given mouse event.
  @param event The mouse event of which the clicked button shal be got.
  @returns {number} The number of the clicked mouse button. (0 : none | 1 : leftButton | 2 : middleButton | 3 : rightButton)