Fix legacy widget block preview iframe in plugin (#32300)

lib/blocks.php

@@ -5,6 +5,24 @@
  * @package gutenberg
  */
 
+/*
+ * Fixes the priority of register_block_core_legacy_widget().
+ *
+ * This hook was incorrectly added to Core with priority 20. #32300 fixes this
+ * but causes block registration warnings in the Gutenberg plugin until the
+ * changes are made in Core.
+ *
+ * This temporary fix can be removed after the changes to
+ * @wordpress/block-library in #32300 have been published to npm and updated in
+ * Core.
+ *
+ * See https://github.com/WordPress/gutenberg/pull/32300.
+ */
+if ( 20 === has_action( 'init', 'register_block_core_legacy_widget' ) ) {
+	remove_action( 'init', 'register_block_core_legacy_widget', 20 );
+	add_action( 'init', 'register_block_core_legacy_widget', 10 );
+}
+
 /**
  * Substitutes the implementation of a core-registered block type, if exists,
  * with the built result from the plugin.

packages/block-library/src/legacy-widget/index.php

@@ -50,21 +50,25 @@ function render_block_core_legacy_widget( $attributes ) {
 }
 
 /**
- * On application init this does two things:
- *
- * - Registers the 'core/legacy-widget' block.
- * - Intercepts any request with legacy-widget-preview in the query param and,
- *   if set, renders a page containing a preview of the requested Legacy Widget
- *   block.
+ * Registers the 'core/legacy-widget' block.
  */
-function init_legacy_widget_block() {
+function register_block_core_legacy_widget() {
 	register_block_type_from_metadata(
 		__DIR__ . '/legacy-widget',
 		array(
 			'render_callback' => 'render_block_core_legacy_widget',
 		)
 	);
+}
+
+add_action( 'init', 'register_block_core_legacy_widget' );
 
+/**
+ * Intercepts any request with legacy-widget-preview in the query param and, if
+ * set, renders a page containing a preview of the requested Legacy Widget
+ * block.
+ */
+function handle_legacy_widget_preview_iframe() {
 	if ( empty( $_GET['legacy-widget-preview'] ) ) {
 		return;
 	}
@@ -110,4 +114,7 @@ function init_legacy_widget_block() {
 	exit;
 }
 
-add_action( 'init', 'init_legacy_widget_block' );
+// Ensure handle_legacy_widget_preview_iframe() is called after Core's
+// register_block_core_legacy_widget() (priority = 10) and after Gutenberg's
+// register_block_core_legacy_widget() (priority = 20).
+add_action( 'init', 'handle_legacy_widget_preview_iframe', 21 );