e reflection purely because there is no purely declarative
         * way of transitioning to/from `height: auto`. This callback manages
         * setting explicit heights for the property so that CSS can interpolate it.
         *
         * @see #_onTransitionEnd
         */
        _collapsedChanged: function() {
          if (!this._isReady) {
            this.toggleAttribute('_collapsed', this.collapsed);
            return;
          }

          var container = this.$.transitionMask;
          var c