Block template resolution isn't working on Windows

[Mamaduka]

Originally reported on Twitter by @gmazzap. Thank you, Giuseppe.

They recursively check the theme's "template/" folder to find HTML templates, but they don't normalize found paths.
So templates inside "template/" subfolders are not found on Windows.

https://twitter.com/gmazzap/status/1496037481069916160

[gmazzap]

Thank you @Mamaduka

First, sorry for the obvious typo in the tweet (it's templates/)

How to reproduce

In a vanilla WP 5.9 installtion, add the following to the twentytwentytwo functions.php:

add_action(
    'theme_page_templates',
    function ($templates): array {
        return array_merge(['my-folder/my-template.php' => 'My Custom Template'], $templates);
    }
);

Then edit/create a page in WP, and select the "My Custom Template" template.

Now, in the twentytwentytwo templates/ folder, create the my-folder/ folder, and add a my-template.html template.
(Maybe you can copy and rename page.html).

In the template editor you now see the "My Custom Template" template. Change something so you can recognize when it is used.

Go back to the edit screen of the page that has assigned the template and preview it.

If you are on Windows, the template will not be used.

Debugging

The issue happens in the gutenberg_get_block_templates / get_block_templates function.

On Windows, here:

gutenberg/lib/compat/wordpress-5.9/block-template-utils.php

Line 705 in 7628871

$should_include = $is_not_custom && $fits_slug_query && $fits_area_query;

$should_include is false, because $fits_slug_query is false.

And that because here

gutenberg/lib/compat/wordpress-5.9/block-template-utils.php

Line 702 in 7628871

! isset( $query['slug__in'] ) || in_array( $template_file['slug'], $query['slug__in'], true );

$query['slug__in'] will contain the array item my-folder/my-template, but $template_file['slug'] is my-folder\my-template.

So the cause is that $template_file['slug'] has Windows directory separator. And that's because _get_block_templates_paths does not normalize the path here:

gutenberg/lib/compat/wordpress-5.9/block-template-utils.php

Line 230 in 7628871

$path_list[] = $path;

Notes

More generally, in the whole Gutenberg code I see there a mix of hardcoding / as separator with using DIRECTORY_SEPARATOR. I think a better approach would be to make sure that anywhere there a path, that is normalized.