Made a small modification to JX.History.install() to get some debugging information that is VERY interesting... Its at the very bottom.

install : function(mechanism) {
  if (__DEV__) {
    if (JX.History._installed) {
      JX.$E('JX.History.install(): can only install once.');
    }
    JX.History._installed = true;
  }

  mechanism = mechanism || JX.History.DEFAULT;

  if (mechanism >= JX.History.PUSHSTATE && 'pushState' in history) {
    JX.History._mechanism = JX.History.PUSHSTATE;
    JX.History._initialPath = JX.History._getBasePath(location.href);
    JX.Stratcom.listen('popstate', null, JX.History._handleChange);
  } else if (mechanism >= JX.History.HASHCHANGE &&
             'onhashchange' in window) {
    JX.History._mechanism = JX.History.HASHCHANGE;
    JX.Stratcom.listen('hashchange', null, JX.History._handleChange);
  } else {
    JX.History._mechanism = JX.History.POLLING;
    setInterval(JX.History._handleChange, 200);
  }
  console.log(JX.History._mechanism);
  console.log(history.state);
},

For Chrome, Firefox, and now even Safari, the value of history.state is sometimes set to an object like { 'quicksand' : 1 }. The mechanism is always PUSHSTATE.

On /p/<username>/, I am getting the following consistently

Safari: { quicksand : 1 }
Chrome: { quicksand : 1 }
Firefox: { quicksand : 3 }

On /, I am getting the following consistently

Safari: { quicksand : 2 }
Chrome: { quicksand : 2 }
Firefox: { quicksand : 1 }

Anyway, just a dump of where I am at on this mystery.

...meant to 'cc' you in case my last comment sparks any debugging juices...

Oh, okay, here's a big break through, at least on reproduction steps.

• open up a new tab in any of these browsers.
• visit phabricator instance home page
  • console.log(history.state); in install will show null
• visit another page
  • console.log(data); in quicksand _onchange event will show {quicksand : 1}
• visit phabricator instance home page again
  • console.log(data); in quicksand _onchange event will show {quicksand : 2}
• reload the page
  • console.log(history.state); in install will show {quicksand : 2}

Things seem to work okay until you reload the page.

If browsers are carrying state across reloads, maybe we just need to initialize JX.Quicksand.id to window.history.state || 0 (and throw away that object patch).