When using a child theme, blocks registered in the parent theme have the wrong asset file path which prevents styles from loading

[jmaasonoffice]

I am registering blocks in a theme using register_block_type() pointing to a block.json. This works fine.

I then made a child theme and noticed that in the frontend the styles are loaded but in the backend all my blocks' styles are missing. This is because the URI for the assets includes the absolute file path, e.g. http://localhost/wp-content/plugins/var/www/html/wp-content/themes/parent/block/style.css. It worked in the frontend, because WordPress was inlining the styles which made correct use of the absolute file path.

The root cause, as far as I can tell, is that blocks.php makes the assumption that all blocks are registered in the currently active theme. In the case of a child theme, this is not necessarily true. Then the check if the path starts with the current theme's path is faulty.

For a bit of context: We want to deliver themes to customers, but keep the option of making individual modifications. For that we plan to use child themes, for example to add additional blocks.

For now, we can work around this issue by registering the blocks styles separately and passing the registered style handle instead of the file path. We can achieve this by hooking into block_type_metadata and replacing the $metadata['style'] and $metadata['editorStyle'] with the registered handle.

However, I believe that this current behavior is confusing and that the code in blocks.php should probably also check for the case that a child theme is active and the blocks are registered in the parent theme.