docs: function pointerDown(e) {
          // don't register an activePointer if more than one touch is active.
          var singleFinger = e.pointerType === POINTER_TYPE_MOUSE ||
              e.pointerType === POINTER_TYPE_PEN ||
              (e.pointerType === POINTER_TYPE_TOUCH && e.isPrimary);
  
          if (!isScrolling && singleFinger) {
              activePointer = {
                  id: e.pointerId,
                  clientX: e.clientX,
                  clientY: e.clientY,
                  x: (e.x || e.pageX),
                  y: (e.y || e.pageY),
                  type: e.pointerType
              };
          }
      }