Widgets: Preserve classic sidebars when switching to a block theme.

When switching to a block theme, classic sidebars were orphaned and their widgets remapping to the `'wp_inactive_widgets'` sidebar . This changeset preserves the sidebars and their widgets, providing a migration path to a block theme without losing the widgets.

Classic sidebars are now:

* Stored in a new theme mod called `'wp_classic_sidebars'`;
* Restored to the `$wp_registered_sidebars` global variable when the `'widgets_init'` action fires (via a new internal function called `_wp_block_theme_register_classic_sidebars()`);
* And marked as `'inactive'` when interacting with sidebars REST API endpoint.

References:
* [https://github.com/WordPress/gutenberg/pull/45509 Gutenberg PR 45509] which adds an option for importing widgets from sidebars into template parts.

Follow-up to [50995], [6334].

Props mamaduka, audrasjb, hellofromTonya, ironprogrammer, jameskoster, joen, matveb, mukesh27, noisysocks, poena, youknowriad.
Fixes #57531.
Built from https://develop.svn.wordpress.org/trunk@55200


git-svn-id: http://core.svn.wordpress.org/trunk@54733 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/default-filters.php

@@ -626,6 +626,7 @@ add_filter( 'nav_menu_css_class', 'wp_nav_menu_remove_menu_item_has_children_cla
 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.

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

@@ -339,6 +339,10 @@ class WP_REST_Sidebars_Controller extends WP_REST_Controller {
 			$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();

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() );
 
 	/**

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55199';
+$wp_version = '6.2-alpha-55200';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

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;
+	}
+}