// Shows the number of new messages in the browser tab title while the tab is in the background.
(function () {
  var box = document.getElementById('chat');
  if (!box) return;
  var title = document.title, unread = 0, seen = 0;

  function ids() {
    return Array.prototype.map.call(box.querySelectorAll('.chat-msg'), function (m) { return +m.dataset.id; });
  }
  seen = Math.max.apply(null, ids().concat(0));

  box.addEventListener('chat:updated', function () {
    var fresh = ids().filter(function (id) { return id > seen; });
    if (!fresh.length) return;
    seen = Math.max.apply(null, fresh.concat(seen));
    if (document.hidden) {
      unread += fresh.length;
      document.title = '(' + unread + ') ' + title;
    }
  });
  document.addEventListener('visibilitychange', function () {
    if (!document.hidden) { unread = 0; document.title = title; }
  });
})();