--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/rest-api/endpoints/class-wp-rest-navigation-fallback-controller.php	Fri Sep 05 18:40:08 2025 +0200
@@ -0,0 +1,191 @@
+<?php
+/**
+ * WP_REST_Navigation_Fallback_Controller class
+ *
+ * REST Controller to create/fetch a fallback Navigation Menu.
+ *
+ * @package WordPress
+ * @subpackage REST_API
+ * @since 6.3.0
+ */
+
+/**
+ * REST Controller to fetch a fallback Navigation Block Menu. If needed it creates one.
+ *
+ * @since 6.3.0
+ */
+class WP_REST_Navigation_Fallback_Controller extends WP_REST_Controller {
+
+	/**
+	 * The Post Type for the Controller
+	 *
+	 * @since 6.3.0
+	 *
+	 * @var string
+	 */
+	private $post_type;
+
+	/**
+	 * Constructs the controller.
+	 *
+	 * @since 6.3.0
+	 */
+	public function __construct() {
+		$this->namespace = 'wp-block-editor/v1';
+		$this->rest_base = 'navigation-fallback';
+		$this->post_type = 'wp_navigation';
+	}
+
+	/**
+	 * Registers the controllers routes.
+	 *
+	 * @since 6.3.0
+	 */
+	public function register_routes() {
+
+		// Lists a single nav item based on the given id or slug.
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base,
+			array(
+				array(
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => array( $this, 'get_item' ),
+					'permission_callback' => array( $this, 'get_item_permissions_check' ),
+					'args'                => $this->get_endpoint_args_for_item_schema( WP_REST_Server::READABLE ),
+				),
+				'schema' => array( $this, 'get_item_schema' ),
+			)
+		);
+	}
+
+	/**
+	 * Checks if a given request has access to read fallbacks.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 * @return true|WP_Error True if the request has read access, WP_Error object otherwise.
+	 */
+	public function get_item_permissions_check( $request ) {
+
+		$post_type = get_post_type_object( $this->post_type );
+
+		// Getting fallbacks requires creating and reading `wp_navigation` posts.
+		if ( ! current_user_can( $post_type->cap->create_posts ) || ! current_user_can( 'edit_theme_options' ) || ! current_user_can( 'edit_posts' ) ) {
+			return new WP_Error(
+				'rest_cannot_create',
+				__( 'Sorry, you are not allowed to create Navigation Menus as this user.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		if ( 'edit' === $request['context'] && ! current_user_can( $post_type->cap->edit_posts ) ) {
+			return new WP_Error(
+				'rest_forbidden_context',
+				__( 'Sorry, you are not allowed to edit Navigation Menus as this user.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		return true;
+	}
+
+	/**
+	 * Gets the most appropriate fallback Navigation Menu.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
+	 */
+	public function get_item( $request ) {
+		$post = WP_Navigation_Fallback::get_fallback();
+
+		if ( empty( $post ) ) {
+			return rest_ensure_response( new WP_Error( 'no_fallback_menu', __( 'No fallback menu found.' ), array( 'status' => 404 ) ) );
+		}
+
+		$response = $this->prepare_item_for_response( $post, $request );
+
+		return $response;
+	}
+
+	/**
+	 * Retrieves the fallbacks' schema, conforming to JSON Schema.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @return array Item schema data.
+	 */
+	public function get_item_schema() {
+		if ( $this->schema ) {
+			return $this->add_additional_fields_schema( $this->schema );
+		}
+
+		$this->schema = array(
+			'$schema'    => 'http://json-schema.org/draft-04/schema#',
+			'title'      => 'navigation-fallback',
+			'type'       => 'object',
+			'properties' => array(
+				'id' => array(
+					'description' => __( 'The unique identifier for the Navigation Menu.' ),
+					'type'        => 'integer',
+					'context'     => array( 'view', 'edit', 'embed' ),
+					'readonly'    => true,
+				),
+			),
+		);
+
+		return $this->add_additional_fields_schema( $this->schema );
+	}
+
+	/**
+	 * Matches the post data to the schema we want.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param WP_Post         $item    The wp_navigation Post object whose response is being prepared.
+	 * @param WP_REST_Request $request Request object.
+	 * @return WP_REST_Response $response The response data.
+	 */
+	public function prepare_item_for_response( $item, $request ) {
+		$data = array();
+
+		$fields = $this->get_fields_for_response( $request );
+
+		if ( rest_is_field_included( 'id', $fields ) ) {
+			$data['id'] = (int) $item->ID;
+		}
+
+		$context = ! empty( $request['context'] ) ? $request['context'] : 'view';
+		$data    = $this->add_additional_fields_to_object( $data, $request );
+		$data    = $this->filter_response_by_context( $data, $context );
+
+		$response = rest_ensure_response( $data );
+
+		if ( rest_is_field_included( '_links', $fields ) || rest_is_field_included( '_embedded', $fields ) ) {
+			$links = $this->prepare_links( $item );
+			$response->add_links( $links );
+		}
+
+		return $response;
+	}
+
+	/**
+	 * Prepares the links for the request.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param WP_Post $post the Navigation Menu post object.
+	 * @return array Links for the given request.
+	 */
+	private function prepare_links( $post ) {
+		return array(
+			'self' => array(
+				'href'       => rest_url( rest_get_route_for_post( $post->ID ) ),
+				'embeddable' => true,
+			),
+		);
+	}
+}