function getWindowScrollTop() {
  var rootWindow = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : window;
  var res = rootWindow.scrollY;

  if (res === void 0) {
    // IE8-11
    res = rootWindow.document.documentElement.scrollTop;
  }

  return res;
}
Returns the document's scrollTop property.

@param {Window} rootWindow
@returns {Number}