code: function() {
          var scroll = this.container.scrollTop(),
              offsets = this.offsets,
              onlyWithin = this.options.onlyWithin,
              threshold = this.options.threshold;
  
          this.markers.each(function(index, marker) {
              marker = $(marker);
  
              var offset = offsets[index],
                  top = offset.top - threshold,
                  bot = offset.top + marker.height() - threshold;
  
              // Scroll is within the marker
              if (
                  (onlyWithin && scroll >= top && scroll <= bot) ||
                  (!onlyWithin && scroll >= top)
              ) {
                  this.activate(marker);
  
              // Scroll went outside the marker
              } else {
                  this.deactivate(marker);
              }
          }.bind(this));
  
          this.fireEvent('scroll');
      }
docs: While the element is being scrolled, notify the targets when a marker is reached.
  
  @private