get the scrollable root element on the page

getScrollable

// get the actually scrollable element
function getScrollable( root ){
  if ( !root.nodeName || !!~["iframe", "#document", "html", "body"].indexOf(root.nodeName.toLowerCase()) ) {
    var doc = (root.contentWindow || root).document || root.ownerDocument || root
    root = /webkit/i.test(navigator.userAgent) || doc.compatMode == "BackCompat"
      ? doc.body
      : doc.documentElement;
  }
  return root
}