code: function(evt)
  	{
  		// Special case for mousemove and mousedown we check the buttons
  		// if it exists because which is 0 even if no button is pressed
  		if ('buttons' in evt && (evt.type == 'mousedown' || evt.type == 'mousemove'))
  		{
  			return evt.buttons == 1;
  		}
  		else if ('which' in evt)
  		{
  	        return evt.which === 1;
  	    }
  		else
  		{
  	        return evt.button === 1;
  	    }
  	}
docs: Function: isLeftMouseButton
  
  Returns true if the left mouse button is pressed for the given event.
  To check if a button is pressed during a mouseMove you should use the
  <mxGraph.isMouseDown> property. Note that this returns true in Firefox
  for control+left-click on the Mac.