Plugin: Deprecate metabox supports, fall back to core

docs/designers-developers/developers/backward-compatibility/deprecations.md

@@ -39,6 +39,16 @@ The Gutenberg project's deprecation policy is intended to support backward compa
 - The PHP function `gutenberg_load_list_reusable_blocks` has been removed.
 - The PHP function `gutenberg_common_scripts_and_styles` has been removed. Use [`wp_common_block_scripts_and_styles`](https://developer.wordpress.org/reference/functions/wp_common_block_scripts_and_styles/) instead.
 - The PHP function `gutenberg_enqueue_registered_block_scripts_and_styles` has been removed. Use [`wp_enqueue_registered_block_scripts_and_styles`](https://developer.wordpress.org/reference/functions/wp_enqueue_registered_block_scripts_and_styles/) instead.
+- The PHP function `gutenberg_meta_box_save` has been removed.
+- The PHP function `gutenberg_meta_box_save_redirect` has been removed.
+- The PHP function `gutenberg_filter_meta_boxes` has been removed.
+- The PHP function `gutenberg_intercept_meta_box_render` has been removed.
+- The PHP function `gutenberg_override_meta_box_callback` has been removed.
+- The PHP function `gutenberg_show_meta_box_warning` has been removed.
+- The PHP function `the_gutenberg_metaboxes` has been removed. Use [`the_block_editor_meta_boxes`](https://developer.wordpress.org/reference/functions/the_block_editor_meta_boxes/) instead.
+- The PHP function `gutenberg_meta_box_post_form_hidden_fields` has been removed. Use [`the_block_editor_meta_box_post_form_hidden_fields`](https://developer.wordpress.org/reference/functions/the_block_editor_meta_box_post_form_hidden_fields/) instead.
+- The PHP function `gutenberg_toggle_custom_fields` has been removed.
+- The PHP function `gutenberg_collect_meta_box_data` has been removed. Use [`register_and_do_post_meta_boxes`](https://developer.wordpress.org/reference/functions/register_and_do_post_meta_boxes/) instead.
 
 ## 4.5.0
 - `Dropdown.refresh()` has been deprecated as the contained `Popover` is now automatically refreshed.

gutenberg.php

@@ -52,8 +52,8 @@ function the_gutenberg_project() {
 	<div class="block-editor gutenberg">
 		<h1 class="screen-reader-text"><?php echo esc_html( $post_type_object->labels->edit_item ); ?></h1>
 		<div id="editor" class="block-editor__container gutenberg__editor"></div>
-		<div id="metaboxes" style="display: none;">
-			<?php the_gutenberg_metaboxes(); ?>
+		<div id="metaboxes" class="hidden">
+			<?php the_block_editor_meta_boxes(); ?>
 		</div>
 	</div>
 	<?php
@@ -246,7 +246,7 @@ function gutenberg_init( $return, $post ) {
 	 * includes/meta-boxes is typically loaded from edit-form-advanced.php.
 	 */
 	require_once ABSPATH . 'wp-admin/includes/meta-boxes.php';
-	gutenberg_collect_meta_box_data();
+	register_and_do_post_meta_boxes( $post );
 
 	require_once ABSPATH . 'wp-admin/admin-header.php';
 	the_gutenberg_project();

lib/client-assets.php

@@ -1028,10 +1028,10 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
 	$meta_box_url = admin_url( 'post.php' );
 	$meta_box_url = add_query_arg(
 		array(
-			'post'           => $post->ID,
-			'action'         => 'edit',
-			'classic-editor' => true,
-			'meta_box'       => true,
+			'post'            => $post->ID,
+			'action'          => 'edit',
+			'meta-box-loader' => true,
+			'_wpnonce'        => wp_create_nonce( 'meta-box-loader' ),
 		),
 		$meta_box_url
 	);