Enable first/last child insertion next to Classic block

src/wp-includes/blocks.php

@@ -1170,6 +1170,25 @@ function apply_block_hooks_to_content_from_post_object( $content, WP_Post $post
 		return apply_block_hooks_to_content( $content, $post, $callback );
 	}
 
+	/*
+	 * If the content was created using the classic editor or using a single Classic block
+	 * (`core/freeform`), it might not contain any block markup at all.
+	 * However, we still might need to inject hooked blocks in the first child or last child
+	 * positions of the parent block. To be able to apply the Block Hooks algorithm, we wrap
+	 * the content in a `core/freeform` wrapper block.
+	 */
+	if ( ! has_blocks( $content ) ) {
+		$original_content = $content;
+
+		$content_wrapped_in_classic_block = get_comment_delimited_block_content(
+			'core/freeform',
+			array(),
+			$content
+		);
+
+		$content = $content_wrapped_in_classic_block;
+	}
+
 	$attributes = array();
 
 	// If context is a post object, `ignoredHookedBlocks` information is stored in its post meta.
@@ -1205,6 +1224,17 @@ function apply_block_hooks_to_content_from_post_object( $content, WP_Post $post
 	// Finally, we need to remove the temporary wrapper block.
 	$content = remove_serialized_parent_block( $content );
 
+	// If we wrapped the content in a `core/freeform` block, we also need to remove that.
+	if ( ! empty( $content_wrapped_in_classic_block ) ) {
+		/*
+		 * We cannot simply use remove_serialized_parent_block() here,
+		 * as that function assumes that the block wrapper is at the top level.
+		 * However, there might now be a hooked block inserted next to it
+		 * (as first or last child of the parent).
+		 */
+		$content = str_replace( $content_wrapped_in_classic_block, $original_content, $content );
+	}
+
 	return $content;
 }