code: function _updateScrollerShadow($displayElement, $scrollElement, $shadowTop, $shadowBottom, isPositionFixed) {
          var offsetTop           = 0,
              scrollElement       = $scrollElement.get(0),
              scrollTop           = scrollElement.scrollTop,
              topShadowOffset     = Math.min(scrollTop - SCROLL_SHADOW_HEIGHT, 0),
              displayElementWidth = $displayElement.width();
  
          if ($shadowTop) {
              $shadowTop.css("background-position", "0px " + topShadowOffset + "px");
  
              if (isPositionFixed) {
                  offsetTop = $displayElement.offset().top;
                  $shadowTop.css("top", offsetTop);
              }
  
              if (isPositionFixed) {
                  $shadowTop.css("width", displayElementWidth);
              }
          }
  
          if ($shadowBottom) {
              var clientHeight        = scrollElement.clientHeight,
                  outerHeight         = $displayElement.outerHeight(),
                  scrollHeight        = scrollElement.scrollHeight,
                  bottomShadowOffset  = SCROLL_SHADOW_HEIGHT; // outside of shadow div viewport
  
              if (scrollHeight > clientHeight) {
                  bottomShadowOffset -= Math.min(SCROLL_SHADOW_HEIGHT, (scrollHeight - (scrollTop + clientHeight)));
              }
  
              $shadowBottom.css("background-position", "0px " + bottomShadowOffset + "px");
              $shadowBottom.css("top", offsetTop + outerHeight - SCROLL_SHADOW_HEIGHT);
              $shadowBottom.css("width", displayElementWidth);
          }
      }
docs: Positions shadow background elements to indicate vertical scrolling.
  @param {!DOMElement} $displayElement the DOMElement that displays the shadow
  @param {!Object} $scrollElement the object that is scrolled
  @param {!DOMElement} $shadowTop div .scroller-shadow.top
  @param {!DOMElement} $shadowBottom div .scroller-shadow.bottom
  @param {boolean} isPositionFixed When using absolute position, top remains at 0.