Fix opinionated block styles loading in editor.

[jffng]

What?

This PR checks if the current theme has declared support for wp-block-styles before adding the dependency in the site editor.

Why?

Right now the opinionated styles are being loaded in the editor, regardless if the theme has declared support.

How?

I traced the problem to the 6.0 compat file.

Testing Instructions

1. Activate emptytheme
2. Go to the site editor, verify theme.css is loaded. A quick visual check is adding a separator at the root of the page and seeing that the width is limited to 100px.
3. Comment out / remove the add_theme_support( 'wp-block-styles' ); line of the theme's functions.php
4. Refresh the site editor, verify the theme.css is no longer loaded. (The separator's width should not be limited.)

Screenshots or screencast

cc @WordPress/block-themers

[mikachan]

This is working well for me following the test instructions. I can see that theme.css is not loaded in the editor unless the theme has opted into wp-block-styles.

[draganescu]

How is this related to the similar code in gutenberg_edit_site_init

if (
		current_theme_supports( 'wp-block-styles' ) ||
		( ! is_array( $editor_styles ) || count( $editor_styles ) === 0 )
	) {
		wp_enqueue_style( 'wp-block-library-theme' );
	}

[jffng]

I am not sure, but I did try changing those conditions (removing the enqueue) and it did not affect whether the theme.css was loaded in the site editor.

[scruffian]

I think its duplicated because this is a compat file, but it introduces this bug into 6.0. IMO this fix should be backported to 6.0.

[scruffian]

cc @adamziel @gziolo

[gziolo]

Added to the 6.0 board in the Triage column. @ndiego will take care of everything else as usual. Thank you.

[ndiego]

I am struggling to replicate the original issue. I am using Gutenberg 13.1 and 6.0 RC1. I have also tested with just RC1. With emptytheme, the separator is 100px. When I comment out that add_theme_support( 'wp-block-styles' );, it goes full width. This is the intended functionality, correct?

[scruffian]

This is the intended functionality, correct?

Correct

[ndiego]

@scruffian, but my testing above was without this PR applied.

[scruffian]

@scruffian, but my testing above was without this PR applied.

Did you test in the Site Editor, not the Post Editor? That's the only environment where I see the issue.

[ndiego]

Did you test in the Site Editor, not the Post Editor? That's the only environment where I see the issue.

🤦‍♂️ My apologies, I was looking in the Post Editor. Yes, I can replicate it in the Site Editor and this PR fixes the issue.

[ndiego]

@gziolo @adamziel this is a pretty important one as it introduces unexpected behavior for themes that have disabled block styles. I recommend we backport this once merged.

[jffng]

Thanks everyone for the reviews and @scruffian for the rebase.

[adamziel]

I went ahead and cherry-picked this PR to the wp/6.0 branch to get it included in the next release: b994942

[gziolo]

This change has to be applied manually in WordPress core.

[gziolo]

Committed to WP Core with WordPress/wordpress-develop@af8844c.