getScrollingElement.js

/*
 * How to detect which element is the scrolling element in charge of scrolling the viewport:
 *
 * - in Quirks mode the scrolling element is the "body"
 * - in Standard mode the scrolling element is the "documentElement"
 *
 * webkit based browsers always use the "body" element, disrespectful of the specifications:
 *
 *  http://dev.w3.org/csswg/cssom-view/#dom-element-scrolltop
 *
 * This feature detection helper allow cross-browser scroll operations on the viewport,
 * it will guess which element to use in each browser both in Quirk and Standard modes.
 * See how this can be used in a "smooth scroll to anchors references" example here:
 *
 *  https://dl.dropboxusercontent.com/u/598365/scrollTo/scrollTo.html
 *
 * It is just a fix for possible differences between browsers versions (currently any Webkit).
 * In case the Webkit bug get fixed someday, it will just work if they follow the specs. Win !
 *
 * Author: Diego Perini
 * Updated: 2014/09/18
 * License: MIT
 *
 */
function getScrollingElement() {
  var d = document;
  return  d.documentElement.scrollHeight > d.body.scrollHeight &&
          d.compatMode.indexOf('CSS1') == 0 ?
          d.documentElement :
          d.body;
}

This code is now being used (with Diego’s permission) in a document.scrollingElement polyfill: https://github.com/mathiasbynens/document.scrollingElement

Actually, I don't understand this logic at all. In most cases in Chrome and Firefox, document.body.scrollHeight == document.documentElement.scrollHeight and so you'll get 'body' even when that's not correct. Other than wacky cases where both html and body have 'overflow' set, I believe these values always both include the document scroll height. They can be different when the body has additional margin/padding but that still doesn't give you any signal about which element is the scroller and so shouldn't be impacting the return value.

Is there a case I'm missing? Can you give me an example where this actually returns 'documentElement'?

Oh, I think I see. This relies on body having some margin. In that case I think it does work. I was testing on a few pages that happened to have 'body {margin: 0};' (not an uncommon pattern) where it falls down.