index.es6.js

if (window.history.pushState) {
  const SCROLL_RESTORATION_TIMEOUT_MS = 3000;
  const TRY_TO_SCROLL_INTERVAL_MS = 50;

  const originalPushState = window.history.pushState;
  const originalReplaceState = window.history.replaceState;

  // Store current scroll position in current state when navigating away.
  window.history.pushState = function() {
    const newStateOfCurrentPage = Object.assign({}, window.history.state, {
      __scrollX: window.scrollX,
      __scrollY: window.scrollY,
    });
    originalReplaceState.call(window.history, newStateOfCurrentPage, '');

    originalPushState.apply(window.history, arguments);
  };

  // Make sure we don't throw away scroll position when calling "replaceState".
  window.history.replaceState = function(state, ...otherArgs) {
    const newState = Object.assign({}, {
      __scrollX: window.history.state && window.history.state.__scrollX,
      __scrollY: window.history.state && window.history.state.__scrollY,
    }, state);

    originalReplaceState.apply(window.history, [newState].concat(otherArgs));
  };

  let timeoutHandle = null;
  let scrollBarWidth = null;

  // Try to scroll to the scrollTarget, but only if we can actually scroll
  // there. Otherwise keep trying until we time out, then scroll as far as
  // we can.
  const tryToScrollTo = (scrollTarget) => {
    // Stop any previous calls to "tryToScrollTo".
    clearTimeout(timeoutHandle);

    const body = document.body;
    const html = document.documentElement;
    if (!scrollBarWidth) {
      scrollBarWidth = getScrollbarWidth();
    }

    // From http://stackoverflow.com/a/1147768
    const documentWidth = Math.max(body.scrollWidth, body.offsetWidth,
      html.clientWidth, html.scrollWidth, html.offsetWidth);
    const documentHeight = Math.max(body.scrollHeight, body.offsetHeight,
      html.clientHeight, html.scrollHeight, html.offsetHeight);

    if (documentWidth + scrollBarWidth - window.innerWidth >= scrollTarget.x &&
        documentHeight + scrollBarWidth - window.innerHeight >= scrollTarget.y ||
        Date.now() > scrollTarget.latestTimeToTry) {
      window.scrollTo(scrollTarget.x, scrollTarget.y);
    } else {
      timeoutHandle = setTimeout(() => tryToScrollTo(scrollTarget),
        TRY_TO_SCROLL_INTERVAL_MS);
    }
  };

  // Try scrolling to the previous scroll position on popstate
  const onPopState = () => {
    const state = window.history.state;

    if (state &&
        Number.isFinite(state.__scrollX) &&
        Number.isFinite(state.__scrollY)) {
      setTimeout(() => tryToScrollTo({
        x: state.__scrollX,
        y: state.__scrollY,
        latestTimeToTry: Date.now() + SCROLL_RESTORATION_TIMEOUT_MS,
      }));
    }
  };

  // Calculating width of browser's scrollbar
  function getScrollbarWidth() {
    let outer = document.createElement("div");
    outer.style.visibility = "hidden";
    outer.style.width = "100px";
    outer.style.msOverflowStyle = "scrollbar";

    document.body.appendChild(outer);

    let widthNoScroll = outer.offsetWidth;
    // force scrollbars
    outer.style.overflow = "scroll";

    // add innerdiv
    let inner = document.createElement("div");
    inner.style.width = "100%";
    outer.appendChild(inner);        

    let widthWithScroll = inner.offsetWidth;

    // remove divs
    outer.parentNode.removeChild(outer);

    return widthNoScroll - widthWithScroll;
  }

  window.addEventListener('popstate', onPopState, true);
}