code: function documentScrollY(targetElement) {
    if (targetElement && (targetElement !== window)) {
      return targetElement.scrollTop;
    }
  
    if (detectedIE10_ && (window.pageYOffset != document.documentElement.scrollTop)) {
      return document.documentElement.scrollTop;
    }
  
    return window.pageYOffset || document.documentElement.scrollTop;
  }
docs: Get the document scroll.
  @param {Element} targetElement - Optional target element.
  @return {number}