yTabIndex: function(tabbables) {
        // Implement a merge sort as Array.prototype.sort does a non-stable sort
        // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
        var len = tabbables.length;
        if (len < 2) {
          return tabbables;
        }
        var pivot = Math.ceil(len / 2);
        var left = this._sortByTabIndex(tabbables.slice(0, pivot));
        var right = this._sortByTabIndex(tabbables.slice(pivot));
        return this._mer