code: function() {
          var options = this.options;
  
          if (options.calculate) {
              this.calculate();
          }
  
          if (!this.active) {
              return;
          }
  
          var eHeight = this.elementHeight,
              eTop = this.elementTop,
              pHeight = this.parentHeight,
              pTop = this.parentTop,
              cssTop = this.initialTop,
              scrollTop = $(window).scrollTop(),
              pos = {},
              x = options.xOffset,
              y = 0;
  
          // Scroll is above the parent, remove pin inline styles
          if (scrollTop < pTop || scrollTop === 0) {
              if (this.pinned) {
                  this.unpin();
              }
  
              return;
          }
  
          // Don't extend out the bottom
          var elementMaxPos = scrollTop + eHeight,
              parentMaxHeight = pHeight + pTop;
  
          // Swap positioning of the fixed menu once it reaches the parent borders
          if (options.fixed) {
              if (elementMaxPos >= parentMaxHeight) {
                  y = 'auto';
  
                  pos.position = 'absolute';
                  pos.bottom = 0;
  
              } else if (scrollTop >= eTop) {
                  y = options.yOffset;
  
                  pos.position = 'fixed';
                  pos.bottom = 'auto';
              }
  
          // Stop positioning absolute menu once it exits the parent
          } else {
              pos.position = 'absolute';
  
              if (elementMaxPos >= parentMaxHeight) {
                  y += (pHeight - eHeight);
  
              } else {
                  y += (scrollTop - pTop) + options.yOffset;
              }
  
              // Don't go lower than default top
              if (cssTop && y < cssTop) {
                  y = cssTop;
              }
          }
  
          pos[options.location] = x;
          pos.top = y;
  
          this.element
              .css(pos)
              .addClass('is-pinned');
  
          this.pinned = true;
  
          this.fireEvent('pinned');
      }
docs: Pin the element along the vertical axis while staying contained within the parent.