Script Loader: Restore init hook so block theme styles load on demand in classic themes

[itzmekhokan]

What

Restores the init priority 8 hook for wp_load_classic_theme_block_styles_on_demand() while keeping the wp_default_styles priority 0 hook added in [61981]. Adds guards so filters and actions are not registered twice when the function runs from both hooks.

Why

In WordPress 7.0, commit [61981] moved wp_load_classic_theme_block_styles_on_demand() from init priority 8 to wp_default_styles priority 0 only. That introduced a regression (#65272):

1. register_core_block_style_handles() runs at init priority 9.
2. It checks wp_should_load_separate_core_block_assets() before calling wp_styles().
3. With only the wp_default_styles hook, those filters are not added until WP_Styles is first constructed.
4. register_core_block_style_handles() returns early, so per-block opinionated styles (blocks/*/theme.css, e.g. wp-block-quote-theme) are never registered or enqueued on the frontend.

Symptom: In classic themes with wp-block-styles support (e.g. Twenty Fifteen), Quote block right alignment works in the editor but the border stays on the left on the frontend because theme.css never loads.

The wp_default_styles hook remains so filters are still present when WP_Styles is constructed before init priority 9 (#64846).

How

• Re-add add_action( 'init', 'wp_load_classic_theme_block_styles_on_demand', 8 ) in default-filters.php.
• Keep add_action( 'wp_default_styles', 'wp_load_classic_theme_block_styles_on_demand', 0 ).
• Use has_filter() / has_action() before adding hooks to avoid duplicate registration.
• Add regression test: Tests_Blocks_ClassicThemeBlockStylesOnDemand::test_register_core_block_style_handles_without_prior_wp_styles.

Trac ticket: https://core.trac.wordpress.org/ticket/65272

Use of AI Tools

AI assistance: Yes
Tool(s): Claude

---

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Props khokansardar, wildworks, westonruter.

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[github-actions]

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[westonruter]

Does this cause a regression in what r61981 was committed to fix Core-64846?

[westonruter]

This doesn't seem it will work properly. The has_filter() function returns false|int, with the int being the priority. Since the priority 0 is falsey, this will still end up adding the filter multiple times. So you'd need to instead do:

Suggested change

	if ( ! has_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true' ) ) {
	if ( false === has_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true' ) ) {

[westonruter]

Suggested change

	if ( ! has_filter( 'should_load_separate_core_block_assets', '__return_true' ) ) {
	if ( false === has_filter( 'should_load_separate_core_block_assets', '__return_true' ) ) {

[westonruter]

Suggested change

	if ( ! has_filter( 'should_load_block_assets_on_demand', '__return_true' ) ) {
	if ( false === has_filter( 'should_load_block_assets_on_demand', '__return_true' ) ) {

[westonruter]

Suggested change

	if ( ! has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ) ) {
	if ( false === has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ) ) {

[itzmekhokan]

Does this cause a regression in what r61981 was committed to fix Core-64846?

Thanks @westonruter — good catch on has_filter() / has_action() returning priority 0, which is falsy. Updated all four checks to use false ===.

Re #64846: this should not regress r61981 because the wp_default_styles priority 0 hook is retained. That hook ensures filters are present when WP_Styles is constructed before init priority 9, so wp-block-library still registers as common.css.

[t-hamano]

Suggested change

	* @since 7.0.0 This is also invoked at the `wp_default_styles` action with priority 0 so that filters are present when
	* @since 7.1.0 This is also invoked at the `wp_default_styles` action with priority 0 so that filters are present when

If this PR can make it into the 7.1 release.

[Copilot]

Pull request overview

This PR restores wp_load_classic_theme_block_styles_on_demand() on init (priority 8) while keeping the wp_default_styles (priority 0) invocation, addressing a regression where classic themes may fail to register/enqueue per-block “theme.css” styles when WP_Styles has not yet been constructed.

Changes:

• Re-adds init (priority 8) hook for wp_load_classic_theme_block_styles_on_demand() alongside the existing wp_default_styles (priority 0) hook.
• Adds duplicate-registration guards inside wp_load_classic_theme_block_styles_on_demand() when invoked from both hooks.
• Adds PHPUnit regression coverage for the “no prior wp_styles() before init” scenario and for duplicate-hook prevention.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/phpunit/tests/blocks/classicThemeBlockStylesOnDemand.php	Adds regression tests for classic-theme block style handle registration timing and for duplicate hook registration prevention.
src/wp-includes/script-loader.php	Updates wp_load_classic_theme_block_styles_on_demand() docs and adds guards to avoid registering filters/actions twice when called from multiple hooks.
src/wp-includes/default-filters.php	Restores the init priority 8 hook while retaining the wp_default_styles priority 0 hook.

Comments suppressed due to low confidence (3)

src/wp-includes/script-loader.php:3720

• Same as above: false === has_filter() will also skip adding __return_true at priority 0 if it exists at a non-zero priority already. If priority 0 is required (per the inline comment), gate on 0 !== has_filter() so the callback is guaranteed to exist at priority 0.

	if ( false === has_filter( 'should_load_separate_core_block_assets', '__return_true' ) ) {
		add_filter( 'should_load_separate_core_block_assets', '__return_true', 0 );
	}

src/wp-includes/script-loader.php:3732

• Same guard issue here: to ensure __return_true is present at priority 0 (not merely present somewhere), compare has_filter() to 0 rather than false.

	if ( false === has_filter( 'should_load_block_assets_on_demand', '__return_true' ) ) {
		add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
	}

src/wp-includes/script-loader.php:3740

• Similarly for actions: false === has_action() will skip adding at the default priority (10) if the same callback was added at any other priority. If the intent is to ensure it exists at priority 10, compare against 10 instead.

	if ( false === has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ) ) {
		add_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' );
	}

---

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (5)

src/wp-includes/script-loader.php:3704

• The duplicate-hook guard for this filter only checks for the callback’s presence anywhere. If a plugin/theme has already added __return_true at a non-zero priority, this will skip adding the priority-0 default, which can change override ordering compared to the documented intent (“do so at priority zero…”). Consider checking specifically for priority 0 instead of strict false.

	if ( false === has_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true' ) ) {
		add_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true', 0 );
	}

src/wp-includes/script-loader.php:3721

• Same issue as above: using false === has_filter() will skip adding the priority-0 default if __return_true is already present at another priority, which can alter opt-out behavior/order. Prefer a priority-0 check (or always add at 0) to preserve the intended default.

	if ( false === has_filter( 'should_load_separate_core_block_assets', '__return_true' ) ) {
		add_filter( 'should_load_separate_core_block_assets', '__return_true', 0 );
	}

src/wp-includes/script-loader.php:3733

• Same as the other guards: this currently skips adding the priority-0 default when __return_true is already present at a different priority, which can change how later overrides behave. Checking for priority 0 keeps the function’s documented priority semantics intact.

	if ( false === has_filter( 'should_load_block_assets_on_demand', '__return_true' ) ) {
		add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
	}

tests/phpunit/tests/blocks/classicThemeBlockStylesOnDemand.php:30

• The tests add wp-block-styles theme support but never remove it, which can leak global theme-support state into subsequent tests. Clean it up in tear_down() (similar to other dependency/theme-support tests).

	public function tear_down() {
		global $wp_styles;
		$wp_styles = $this->original_wp_styles;

		parent::tear_down();
	}

tests/phpunit/tests/blocks/classicThemeBlockStylesOnDemand.php:23

• These tests assume a classic (non-block) theme; wp_load_classic_theme_block_styles_on_demand() bails early for block themes. To keep the tests isolated from other tests that may have switched themes, set the theme explicitly in set_up().

	public function set_up() {
		parent::set_up();

		global $wp_styles;
		$this->original_wp_styles = $wp_styles;
	}