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Props westonruter, peterwilsoncc.

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Test this pull request with WordPress Playground.

On this branch, I'm seeing the inline block styles loading when running the testing plugin included on the ticket.

Reproduction steps:

1. Activate 2014
2. Deactivate the test plugin
3. Create a post with the gallery block or any other block that contains styles
4. Activate the test plugin
5. Check out the 6.8 branch (no need to npm install or rebuild assets)
6. Load the post created earlier
7. The gallery block will not be styled as there are are no block styles or inline CSS.
8. Check out this branch
9. Reload the post created earlier
10. The gallery block will be styled as the inline block styles are included in the output

I'm figuring out a test for this, will follow up once I have one.

@peterwilsoncc as we've chatted about over Slack, the issue is there are two test plugins:

1. Enqueue Stylesheet at WP Footer (as listed in the ticket description)
2. Disable Block Styles (listed in comment)

To test the issue with a theme not wanting to have any block styles at all, the second plugin needs to be used to test. The first plugin is just to test that late-enqueued styles successfully get printed even when the wp-block-library is absent (and thus the normal placeholder in the inline style is not present, meaning a fallback to inserting the late-printed styles before </head> is needed.

As discussed in Slack, I think I was using the wrong plugin when testing earlier.

At the time of my earlier tests I was running the following, which produces different results on each branch.

add_action(
	'enqueue_block_assets',
	function (): void {
		wp_deregister_style( 'wp-block-library' );
	}
);

add_action(
	'wp_head',
	static function () {
		echo '<style>body { background-color: red; }</style>';
	}
);

add_action(
	'wp_footer',
	static function () {
		wp_register_style( 'lime-background', null );
		wp_add_inline_style( 'lime-background', 'body { background-color: lime !important; }' );
		wp_enqueue_style( 'lime-background' );
	}
);

Using the second plugin on the ticket I am seeing the same thing on both branches, IE running

add_action(
        'enqueue_block_assets',
        function (): void {
                wp_deregister_style( 'wp-block-library' );
                wp_register_style( 'wp-block-library', '' );
        }
);

add_action(
        'after_setup_theme',
        static function (): void {
                remove_theme_support( 'wp-block-styles' );
        },
        PHP_INT_MAX
);

So I think this is good to go.

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61076
GitHub commit: d14ae9c

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.

Committed in r61076.