Site Editor: Load as full screen by default.

To avoid UI jumps when loading the page, the site-editor.php page
is marked as full screen by default. 
This also matches the behavior of the post editor.

See #54337.

Built from https://develop.svn.wordpress.org/trunk@52071


git-svn-id: http://core.svn.wordpress.org/trunk@51663 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/site-editor.php

@@ -31,6 +31,14 @@ $parent_file = 'themes.php';
 $current_screen = get_current_screen();
 $current_screen->is_block_editor( true );
 
+// Default to is-fullscreen-mode to avoid jumps in the UI.
+add_filter(
+	'admin_body_class',
+	static function( $classes ) {
+		return "$classes is-fullscreen-mode";
+	}
+);
+
 $block_editor_context = new WP_Block_Editor_Context();
 
 $active_global_styles_id = WP_Theme_JSON_Resolver::get_user_custom_post_type_id();

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52070';
+$wp_version = '5.9-alpha-52071';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.