WP_Customize_Nav_Menu_Setting::preview

	 */
	public function preview() {
		if ( $this->is_previewed ) {
			return false;
		}

		$undefined      = new stdClass();
		$is_placeholder = ( $this->term_id < 0 );
		$is_dirty       = ( $undefined !== $this->post_value( $undefined ) );
		if ( ! $is_placeholder && ! $is_dirty ) {
			return false;
		}

		$this->is_previewed       = true;
		$this->_original_value    = $this->value();
		$this->_previewed_blog_id = get_current_blog_id();

		add_filter( 'wp_get_nav_menus', array( $this, 'filter_wp_get_nav_menus' ), 10, 2 );
		add_filter( 'wp_get_nav_menu_object', array( $this, 'filter_wp_get_nav_menu_object' ), 10, 2 );
		add_filter( 'default_option_nav_menu_options', array( $this, 'filter_nav_menu_options' ) );
		add_filter( 'option_nav_menu_options', array( $this, 'filter_nav_menu_options' ) );

		return true;