Backport: Store legacy sidebars when switching to a block theme

src/wp-includes/default-filters.php

@@ -623,6 +623,7 @@
 add_action( 'after_setup_theme', 'wp_setup_widgets_block_editor', 1 );
 add_action( 'init', 'wp_widgets_init', 1 );
 add_action( 'change_locale', array( 'WP_Widget_Media', 'reset_default_labels' ) );
+add_action( 'widgets_init', '_wp_block_theme_register_classic_sidebars', 1 );
 
 // Admin Bar.
 // Don't remove. Wrong way to disable.

src/wp-includes/rest-api/endpoints/class-wp-rest-sidebars-controller.php

@@ -339,6 +339,10 @@ public function prepare_item_for_response( $item, $request ) {
 			$sidebar['class']       = '';
 		}
 
+		if ( wp_is_block_theme() ) {
+			$sidebar['status'] = 'inactive';
+		}
+
 		$fields = $this->get_fields_for_response( $request );
 		if ( rest_is_field_included( 'widgets', $fields ) ) {
 			$sidebars = wp_get_sidebars_widgets();

src/wp-includes/theme.php

@@ -733,11 +733,12 @@ function locale_stylesheet() {
  * @global array                $wp_theme_directories
  * @global WP_Customize_Manager $wp_customize
  * @global array                $sidebars_widgets
+ * @global array                $wp_registered_sidebars
  *
  * @param string $stylesheet Stylesheet name.
  */
 function switch_theme( $stylesheet ) {
-	global $wp_theme_directories, $wp_customize, $sidebars_widgets;
+	global $wp_theme_directories, $wp_customize, $sidebars_widgets, $wp_registered_sidebars;
 
 	$requirements = validate_theme_requirements( $stylesheet );
 	if ( is_wp_error( $requirements ) ) {
@@ -814,6 +815,11 @@ function switch_theme( $stylesheet ) {
 		}
 	}
 
+	// Stores classic sidebars for later use by block themes.
+	if ( $new_theme->is_block_theme() ) {
+		set_theme_mod( 'wp_classic_sidebars', $wp_registered_sidebars );
+	}
+
 	update_option( 'theme_switched', $old_theme->get_stylesheet() );
 
 	/**

src/wp-includes/widgets.php

@@ -2105,3 +2105,29 @@ function wp_check_widget_editor_deps() {
 		}
 	}
 }
+
+/**
+ * Registers the previous theme's sidebars for the block themes.
+ *
+ * @since 6.2.0
+ * @access private
+ *
+ * @global array $wp_registered_sidebars Registered sidebars.
+ */
+function _wp_block_theme_register_classic_sidebars() {
+	global $wp_registered_sidebars;
+
+	if ( ! wp_is_block_theme() ) {
+		return;
+	}
+
+	$classic_sidebars = get_theme_mod( 'wp_classic_sidebars' );
+	if ( empty( $classic_sidebars ) ) {
+		return;
+	}
+
+	// Don't use `register_sidebar` since it will enable the `widgets` support for a theme.
+	foreach ( $classic_sidebars as $sidebar ) {
+		$wp_registered_sidebars[ $sidebar['id'] ] = $sidebar;
+	}
+}

tests/phpunit/tests/rest-api/rest-sidebars-controller.php

@@ -860,6 +860,73 @@ public function test_get_items_inactive_widgets() {
 		);
 	}
 
+	/**
+	 * @ticket 57531
+	 * @covers WP_Test_REST_Sidebars_Controller::prepare_item_for_response
+	 */
+	public function test_prepare_item_for_response_to_set_inactive_on_theme_switch() {
+		$request = new WP_REST_Request( 'GET', '/wp/v2/sidebars/sidebar-1' );
+
+		// Set up the test.
+		wp_widgets_init();
+		$this->setup_widget(
+			'widget_rss',
+			1,
+			array(
+				'title' => 'RSS test',
+			)
+		);
+		$this->setup_widget(
+			'widget_text',
+			1,
+			array(
+				'text' => 'Custom text test',
+			)
+		);
+		$this->setup_sidebar(
+			'sidebar-1',
+			array(
+				'name' => 'Sidebar 1',
+			),
+			array( 'text-1', 'rss-1' )
+		);
+
+		// Validate the state before a theme switch.
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$data     = $this->remove_links( $data );
+
+		$this->assertSame( 'active', $data['status'] );
+		$this->assertFalse(
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'wp_classic_sidebars theme mod should not exist before switching to block theme'
+		);
+
+		switch_theme( 'block-theme' );
+		wp_widgets_init();
+
+		// Validate the state after a theme switch.
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$data     = $this->remove_links( $data );
+
+		$this->assertSame(
+			'inactive',
+			$data['status'],
+			'Sidebar status should have changed to inactive'
+		);
+		$this->assertSame(
+			array( 'text-1', 'rss-1' ),
+			$data['widgets'],
+			'The text and rss widgets should still in sidebar-1'
+		);
+		$this->assertArrayHasKey(
+			'sidebar-1',
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'sidebar-1 should be in "wp_classic_sidebars" theme mod'
+		);
+	}
+
 	/**
 	 * @ticket 41683
 	 */

tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php

@@ -0,0 +1,89 @@
+<?php
+/**
+ * Tests for _wp_block_theme_register_classic_sidebars().
+ *
+ * @group widgets
+ * @covers ::_wp_block_theme_register_classic_sidebars
+ */
+class Tests_Widgets_WpBlockThemeRegisterClassicSidebars extends WP_UnitTestCase {
+	/**
+	 * Original global $wp_registered_sidebars.
+	 *
+	 * @var array
+	 */
+	private static $wp_registered_sidebars;
+
+	public static function set_up_before_class() {
+		global $wp_registered_sidebars;
+		parent::set_up_before_class();
+
+		// Store the original global before running tests.
+		static::$wp_registered_sidebars = $wp_registered_sidebars;
+	}
+
+	public function tear_down() {
+		// Restore the global after each test.
+		global $wp_registered_sidebars;
+		$wp_registered_sidebars = static::$wp_registered_sidebars;
+
+		parent::tear_down();
+	}
+
+	public function test_a_sidebar_should_be_registered() {
+		global $wp_registered_sidebars;
+
+		$sidebar_id = array_key_first( $wp_registered_sidebars );
+		$this->assertNotEmpty( $sidebar_id );
+	}
+
+	/**
+	 * @ticket 57531
+	 */
+	public function test_should_reregister_previous_theme_sidebar() {
+		global $wp_registered_sidebars;
+
+		$sidebar_id = array_key_first( $wp_registered_sidebars );
+
+		switch_theme( 'block-theme' );
+		unregister_sidebar( $sidebar_id );
+
+		// Test before.
+		$this->assertArrayNotHasKey(
+			$sidebar_id,
+			$wp_registered_sidebars,
+			'Sidebar should not be in registered sidebars after unregister'
+		);
+
+		_wp_block_theme_register_classic_sidebars();
+
+		// Test after.
+		$this->assertArrayHasKey(
+			$sidebar_id,
+			$wp_registered_sidebars,
+			'Sidebar should be in registered sidebars after invoking _wp_block_theme_register_classic_sidebars()'
+		);
+	}
+
+	/**
+	 * @ticket 57531
+	 */
+	public function test_should_bail_out_when_theme_mod_is_empty() {
+		global $wp_registered_sidebars;
+
+		// Test state before invoking.
+		$this->assertFalse(
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'Theme mod should not be set before invoking _wp_block_theme_register_classic_sidebars()'
+		);
+
+		$before = $wp_registered_sidebars;
+		_wp_block_theme_register_classic_sidebars();
+
+		// Test state after invoking.
+		$this->assertSameSetsWithIndex(
+			$before,
+			$wp_registered_sidebars,
+			'No change should happen after invoking _wp_block_theme_register_classic_sidebars()'
+		);
+	}
+}