Provide a way to manage wp_navigation posts from wp-admin

lib/navigation.php

@@ -418,13 +418,13 @@ function gutenberg_register_navigation_post_type() {
 		'description'           => __( 'Navigation menus.', 'gutenberg' ),
 		'public'                => false,
 		'has_archive'           => false,
-		'show_ui'               => false,
+		'show_ui'               => true,
 		'show_in_menu'          => 'themes.php',
 		'show_in_admin_bar'     => false,
 		'show_in_rest'          => true,
 		'map_meta_cap'          => true,
 		'rest_base'             => 'navigation',
-		'rest_controller_class' => 'WP_REST_Posts_Controller',
+		'rest_controller_class' => WP_REST_Posts_Controller::class,
 		'supports'              => array(
 			'title',
 			'editor',
@@ -435,3 +435,60 @@ function gutenberg_register_navigation_post_type() {
 	register_post_type( 'wp_navigation', $args );
 }
 add_action( 'init', 'gutenberg_register_navigation_post_type' );
+
+/**
+ * Disable block editor for wp_navigation type posts so they can be managed via the UI.
+ *
+ * @param bool   $value Whether the CPT supports block editor or not.
+ * @param string $post_type Post type.
+ *
+ * @return bool
+ */
+function gutenberg_disable_block_editor_for_navigation_post_type( $value, $post_type ) {
+	if ( 'wp_navigation' === $post_type ) {
+		return false;
+	}
+
+	return $value;
+}
+
+add_filter( 'use_block_editor_for_post_type', 'gutenberg_disable_block_editor_for_navigation_post_type', 10, 2 );
+
+/**
+ * This callback disables content editor for wp_navigation type posts.
+ * Content editor cannot handle wp_navigation type posts correctly.
+ *
+ * @param WP_Post $post An instance of WP_Post class.
+ */
+function gutenberg_disable_content_editor_for_navigation_post_type( $post ) {
+	$post_type = get_post_type( $post );
+	if ( 'wp_navigation' !== $post_type ) {
+		return;
+	}
+
+	remove_post_type_support( $post_type, 'editor' );
+}
+
+add_action( 'edit_form_after_title', 'gutenberg_disable_content_editor_for_navigation_post_type', 10, 1 );
+
+/**
+ * Fixes the label of the 'wp_navigation' admin menu entry.
+ */
+function gutenberg_fix_navigation_items_admin_menu_entry() {
+	global $submenu;
+	if ( ! isset( $submenu['themes.php'] ) ) {
+		return;
+	}
+	$post_type = get_post_type_object( 'wp_navigation' );
+	if ( ! $post_type ) {
+		return;
+	}
+	foreach ( $submenu['themes.php'] as $key => $submenu_entry ) {
+		if ( $post_type->labels->all_items === $submenu['themes.php'][ $key ][0] ) {
+			$submenu['themes.php'][ $key ][0] = $post_type->labels->menu_name; // phpcs:ignore WordPress.WP.GlobalVariablesOverride
+			return;
+		}
+	}
+}
+
+add_action( 'admin_menu', 'gutenberg_fix_navigation_items_admin_menu_entry' );

phpunit/navigation-test.php

@@ -0,0 +1,62 @@
+<?php
+/**
+ * This class is supposed to test the functionality of the navigation.php
+ *
+ * @package Gutenberg
+ */
+
+class WP_Navigation_Test extends WP_UnitTestCase {
+	const NAVIGATION_POST_TYPE     = 'wp_navigation';
+	const NON_NAVIGATION_POST_TYPE = 'wp_non_navigation';
+
+	public function tearDown() {
+		add_post_type_support( static::NAVIGATION_POST_TYPE, 'editor' );
+	}
+
+	public function test_it_doesnt_disable_block_editor_for_non_navigation_post_types() {
+		$filtered_result = gutenberg_disable_block_editor_for_navigation_post_type( true, static::NON_NAVIGATION_POST_TYPE );
+		$this->assertTrue( $filtered_result );
+	}
+
+	public function test_it_disables_block_editor_for_navigation_post_types() {
+		$filtered_result = gutenberg_disable_block_editor_for_navigation_post_type( true, static::NAVIGATION_POST_TYPE );
+		$this->assertFalse( $filtered_result );
+	}
+
+	public function test_it_doesnt_disable_content_editor_for_non_navigation_type_posts() {
+		$post = $this->create_non_navigation_post();
+		$this->assertTrue( $this->supports_block_editor() );
+
+		gutenberg_disable_content_editor_for_navigation_post_type( $post );
+
+		$this->assertTrue( $this->supports_block_editor() );
+	}
+
+	public function test_it_disables_content_editor_for_navigation_type_posts() {
+		$post = $this->create_navigation_post();
+		$this->assertTrue( $this->supports_block_editor() );
+
+		gutenberg_disable_content_editor_for_navigation_post_type( $post );
+
+		$this->assertFalse( $this->supports_block_editor() );
+	}
+
+	private function create_post( $type ) {
+		$post            = new WP_Post( new StdClass() );
+		$post->post_type = $type;
+		$post->filter    = 'raw';
+		return $post;
+	}
+
+	private function create_non_navigation_post() {
+		return $this->create_post( static::NON_NAVIGATION_POST_TYPE );
+	}
+
+	private function create_navigation_post() {
+		return $this->create_post( static::NAVIGATION_POST_TYPE );
+	}
+
+	private function supports_block_editor() {
+		return post_type_supports( static::NAVIGATION_POST_TYPE, 'editor' );
+	}
+}