Fix up output so that position classes are applied to the wrapper

lib/block-supports/layout.php

@@ -305,7 +305,7 @@ function gutenberg_get_classnames_from_last_tag( $html ) {
 		$last_classnames = $tags->get_attribute( 'class' );
 	}
 
-	return $last_classnames;
+	return (string) $last_classnames;
 }
 
 /**
@@ -445,7 +445,7 @@ function gutenberg_render_layout_support_flag( $block_content, $block ) {
 		$content->add_class( implode( ' ', $class_names ) );
 	}
 
-	return $content;
+	return (string) $content;
 }
 
 // Register the block support. (overrides core one).
@@ -559,7 +559,7 @@ function gutenberg_render_layout_position_support( $block_content, $block ) {
 		$content = new WP_HTML_Tag_Processor( $block_content );
 		$content->next_tag();
 		$content->add_class( $class_name );
-		return $content;
+		return (string) $content;
 	}
 	return $block_content;
 }

packages/block-editor/src/hooks/layout.js

@@ -403,16 +403,18 @@ export const withLayoutStyles = createHigherOrderComponent(
 		}
 
 		// Attach a `wp-container-` id-based class name as well as a layout class name such as `is-layout-flex`.
-		const className = classnames(
-			props?.className,
+		const combinedLayoutClassNames = classnames(
 			{
 				[ `wp-container-${ id }` ]: shouldRenderLayoutStyles && !! css, // Only attach a container class if there is generated CSS to be attached.
-				[ `wp-container-${ positionId }` ]:
-					shouldRenderLayoutStyles && !! positionCss, // Use separate container class for position styles in prep for layout styles moving to inner wrapper in: https://github.com/WordPress/gutenberg/pull/44600
 			},
 			layoutClasses
 		);
 
+		const wrapperClassNames = classnames( props?.className, {
+			[ `wp-container-${ positionId }` ]:
+				shouldRenderLayoutStyles && !! positionCss, // Use separate container class for position styles in prep for layout styles moving to inner wrapper in: https://github.com/WordPress/gutenberg/pull/44600
+		} );
+
 		return (
 			<>
 				{ shouldRenderLayoutStyles &&
@@ -430,7 +432,8 @@ export const withLayoutStyles = createHigherOrderComponent(
 					) }
 				<BlockListBlock
 					{ ...props }
-					__unstableLayoutClassNames={ className }
+					className={ wrapperClassNames }
+					__unstableLayoutClassNames={ combinedLayoutClassNames }
 				/>
 			</>
 		);