 observer: '_descriptorChanged',
          },

          /**
           * Whether the property should show a one-liner, or full summary.
           *
           * Note that this property _is_ reflected as an attribute, but we perform
           * the reflection manually. In order to support the CSS transitions, we
           * must calculate the element height before setting the attribute.
           */
          collapsed: {
            type:     Boolean,
            value:    false,
            observer: 