function clamp(pos) {
        pos.top = Math.max(Math.min(pos.top, bounds.bottom - containerNode.offsetHeight), bounds.top);
        pos.left = Math.max(Math.min(pos.left, bounds.right - containerNode.offsetWidth), bounds.left);
      }
Clamps the repositioning of the menu within the confines of
bounding element (often the screen/body)