Move wp_load_classic_theme_block_styles_on_demand() from the init action to wp_default_styles

[westonruter]

Trac ticket: https://core.trac.wordpress.org/ticket/64846

The wp_load_classic_theme_block_styles_on_demand() function was added to run at init action with priority 8 in r61008. This turns out to be too late since styles can be registered earlier. Registering a style earlier causes wp_default_styles() to be called, which registers the wp-block-library style. When a classic theme is active, before wp_load_classic_theme_block_styles_on_demand() is run, the wp_should_load_block_assets_on_demand() function will return false. This results in the combined block library stylesheet being incorrectly registered:

wordpress-develop/src/wp-includes/script-loader.php

Lines 1819 to 1823 in 2bb252a

	$path = "/wp-includes/css/dist/$package/style$suffix.css";
	if ( 'block-library' === $package && wp_should_load_separate_core_block_assets() ) {
	$path = "/wp-includes/css/dist/$package/common$suffix.css";
	}

The point of wp_load_classic_theme_block_styles_on_demand() is to opt-in to loading separate block styles on demand, but at the point it runs it is already too late since the combined wp-block-library has already been registered.

If the init priority of wp_load_classic_theme_block_styles_on_demand() is changed to -1 then this fixes the problem, but this is not the ideal solution since a theme/plugin could always register a style earlier during the init action (e.g. PHP_INT_MIN). Note that the init action is the earliest point to register a style safely since _wp_scripts_maybe_doing_it_wrong() will issue a warning otherwise. Nevertheless, the better solution would be to not use the init action at all, but rather to use the wp_default_styles action (with a low priority) so that we'll be better guaranteed that the filters will have been added.

The issue can be reproduced with the following example plugin:

<?php
/**
 * Plugin Name: Register test style at early init.
 * Plugin URI: https://core.trac.wordpress.org/ticket/64846
 */

add_action(
	'init',
	function () {
		wp_register_style( 'test-early-init', false );
		wp_add_inline_style( 'test-early-init', '/* With a classic theme active, make sure wp-block-library is common.css and not style.css */' );
		wp_enqueue_style( 'test-early-init' );
	},
	0
);

Before ❌

<link rel='stylesheet' id='wp-block-library-css' href='http://localhost:8000/wp-includes/css/dist/block-library/style.css?ver=7.0-beta4-61919-src' media='all' />

<style id="wp-block-accordion-inline-css">
.wp-block-accordion {
  box-sizing: border-box;
}
/*# sourceURL=http://localhost:8000/wp-includes/blocks/accordion/style.css */
</style>

After ✅

<style id="wp-block-library-inline-css">
/**
 * Colors
 */
...

/*# sourceURL=/wp-includes/css/dist/block-library/common.css */
</style>
<style id="wp-block-archives-inline-css">
.wp-block-archives {
  box-sizing: border-box;
}

.wp-block-archives-dropdown label {
  display: block;
}
/*# sourceURL=http://localhost:8000/wp-includes/blocks/archives/style.css */
</style>

Use of AI Tools

None 🧠

---

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[github-actions]

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[Copilot]

Pull request overview

Adjusts when classic themes opt into on-demand block styles (and related output-buffer enhancements) by moving the setup hook earlier in the styles bootstrapping flow, and updates the related PHPUnit test setup ordering.

Changes:

• Move wp_load_classic_theme_block_styles_on_demand() from an init hook to wp_default_styles (priority 0).
• Document the required call ordering for wp_load_classic_theme_block_styles_on_demand().
• Run per-test $set_up closures earlier in test_wp_hoist_late_printed_styles().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/phpunit/tests/template.php	Runs test case setup earlier to align with the updated initialization timing.
src/wp-includes/script-loader.php	Adds documentation about required ordering for classic-theme on-demand block styles initialization.
src/wp-includes/default-filters.php	Changes the default hook timing for enabling classic-theme on-demand block styles.

---

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[adamsilverstein]

Nice!

[github-actions]

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

SVN changeset: 61981
GitHub commit: f29f594

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