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_legacy_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

@@ -737,7 +737,7 @@ function locale_stylesheet() {
  * @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 +814,11 @@ function switch_theme( $stylesheet ) {
 		}
 	}
 
+	// Store legacy sidebars for later use by block themes.
+	if ( $new_theme->is_block_theme() ) {
+		set_theme_mod( 'wp_legacy_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() {
 		}
 	}
 }
+
+/**
+ * Register 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_legacy_sidebars() {
+	global $wp_registered_sidebars;
+
+	if ( ! wp_is_block_theme() ) {
+		return;
+	}
+
+	$legacy_sidebars = get_theme_mod( 'wp_legacy_sidebars' );
+	if ( empty( $legacy_sidebars ) ) {
+		return;
+	}
+
+	// Don't use `register_sidebar` since it will enable the `widgets` support for a theme.
+	foreach ( $legacy_sidebars as $sidebar ) {
+		$wp_registered_sidebars[ $sidebar['id'] ] = $sidebar;
+	}
+}