Template Editing: Templates do not recognize theme template parts without a theme attribute

[kjellr]

A theme attribute is not required for template parts to render in the site editor, however it is required in the template editor.

The fallback hierarchy here should be the same, so that the Template editor knows to look for templates in the theme when there's no theme slug provided.

To reproduce:

The following uses Twenty Twenty-Two as an example, but this should be the case with any block theme that uses a template part in its templates.

1. Use the function from Add an API to define the default template used for the template mode #32771 to map the default Template to one of your theme's existing block templates (one that includes a template part).

/**
 * Add a default block template.
 */
function twentytwentytwo_default_block_template() {
	add_filter( 'block_editor_settings_all',
		function( $settings ) {
			$settings['defaultBlockTemplate'] = file_get_contents( get_theme_file_path( 'block-templates/single.html' ) );
			return $settings;
		}
	);
}
add_action( 'after_setup_theme', 'twentytwentytwo_default_block_template', 0 );

2. Open a post or page in the editor, and select "New" under the Template panel:

3. Note that if your theme uses a theme slug to point to its template part, the template part will appear. Otherwise it will show a "Template not found" error.

Screenshots:

With theme attribute	Without theme attribute

[Mamaduka]

Here's a workaround for now:

/**
 * Add a default block template.
 */
function twentytwentytwo_default_block_template( $settings ) {
	$template_content                 = file_get_contents( get_theme_file_path( 'block-templates/single.html' ) );
	$settings['defaultBlockTemplate'] = _gutenberg_inject_theme_attribute_in_content( $template_content );

	return $settings;
}
add_filter( 'block_editor_settings_all', 'twentytwentytwo_default_block_template' );

[kjellr]

@jffng / @carolinan what do you think about that workaround above? If that is generally ok then we can use it for Twenty Twenty-Two. Otherwise I think we need to have this issue prioritized for 5.9.

[jffng]

It works, though it looks like _gutenberg_inject_theme_attribute_in_content was renamed to _inject_theme_attribute_in_block_template_content in case you want to test it out.

what do you think about that workaround above?

I don't like the idea of adding this to the theme, since the utility function comes from this file which states:

Temporary compatibility shims for features present in Gutenberg.
This file should be removed when WordPress 5.9.0 becomes the lowest
supported version by this plugin.

[Mamaduka]

You should use _inject_theme_attribute_in_block_template_content in theme. But I agree it’s private utility and third parties shouldn’t depend on it.

[kjellr]

Ok, thanks for weighing in. @noisysocks — what do you think about prioritizing this for 5.9? It's preventing block themes from using theme-supplied template parts in the template editor.

[noisysocks]

I'll add it to the board as a general bug to fix (not a blocker).

[Mamaduka]

I'm removing this issue from WP 5.9 project board since RC1 was released last night.

We can add the defaultTemplate option to the theme.json and handle theme attribute injection internally. Probably task for the next major release since we have a workaround.