Site Editor - prevent loading state from showing the admin menu. (#36455

)

* start with is-fullscreen-mode class on the body

* only add filter if loading edit site page

* remove unnecessary static prefix

* Update lib/full-site-editing/edit-site-page.php

lib/full-site-editing/edit-site-page.php

@@ -139,6 +139,23 @@ function gutenberg_edit_site_init( $hook ) {
 		return;
 	}
 
+	// Default to is-fullscreen-mode to avoid rendering wp-admin navigation menu while loading and
+	// having jumps in the UI.
+	add_filter(
+		'admin_body_class',
+		static function( $classes ) {
+			return "$classes is-fullscreen-mode";
+		}
+	);
+
+	/**
+	 * Make the WP Screen object aware that this is a block editor page.
+	 * Since custom blocks check whether the screen is_block_editor,
+	 * this is required for custom blocks to be loaded.
+	 * See wp_enqueue_registered_block_scripts_and_styles in wp-includes/script-loader.php
+	 */
+	$current_screen->is_block_editor( true );
+
 	$custom_settings = array(
 		'siteUrl'                              => site_url(),
 		'postsPerPage'                         => get_option( 'posts_per_page' ),