[dhsysusbsjsi]

reading the source it looks like for some browsers that rate limit url updates, it has to use a different way that nukes your back button ability.

[senfiaj]

function drawWorld() { var hash = '#|' + gridString() + '|[score:' + currentScore() + ']';

  if (urlRevealed) {
    // Use the original game representation on the on-DOM view, as there are no
    // escaping issues there.
    $('#url').textContent = location.href.replace(/#.*$/, '') + hash;
  }

  // Modern browsers escape whitespace characters on the address bar URL for
  // security reasons. In case this browser does that, replace the empty Braille
  // character with a non-whitespace (and hopefully non-intrusive) symbol.
  if (whitespaceReplacementChar) {
    hash = hash.replace(/\u2800/g, whitespaceReplacementChar);
  }

  history.replaceState(null, null, hash);

  // Some browsers have a rate limit on history.replaceState() calls, resulting
  // in the URL not updating at all for a couple of seconds. In those cases,
  // location.hash is updated directly, which is unfortunate, as it causes a new
  // navigation entry to be created each time, effectively hijacking the user's
  // back button.
  if (decodeURIComponent(location.hash) !== hash) {
    console.warn(
      'history.replaceState() throttling detected. Using location.hash fallback'
    );
    location.hash = hash;
  }
}