  container.style.height = fullHeight + 'px'; // Height 'auto'.
          } else {
            container.style.height = ''; // Height specified by CSS rule.
          }

          // We must wait a frame so that the transition engine has a chance to know
          // that something actually changed.
          requestAnimationFrame(function() {
            this.toggleAttribute('_collapsed', collapsed);
            if (this.collapsed) {
              container.style.height = ''; // Height specified by CSS rul