Script Loader: Restore init hook so block theme styles load on demand in classic themes by itzmekhokan · Pull Request #11887 · WordPress/wordpress-develop

itzmekhokan · 2026-05-20T04:45:32Z

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):

register_core_block_style_handles() runs at init priority 9.
It checks wp_should_load_separate_core_block_assets() before calling wp_styles().
With only the wp_default_styles hook, those filters are not added until WP_Styles is first constructed.
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

…mand. Register on-demand block asset filters at init priority 8 again so register_core_block_style_handles() can opt in before WP_Styles is constructed. Keep the wp_default_styles hook from [61981] for early WP_Styles instantiation and guard against duplicate hook registration. Fixes #65272.

…mand. Props Khokan Sardar. Fixes #65272.

github-actions · 2026-05-20T04:45:43Z

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props khokansardar, westonruter.

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github-actions · 2026-05-20T04:50:17Z

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westonruter · 2026-05-21T05:53:34Z

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

westonruter · 2026-05-21T05:57:05Z

 	 * wish to stream responses can more easily turn this off.
 	 */
-	add_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true', 0 );
+	if ( ! has_filter( 'wp_should_output_buffer_template_for_enhancement', '__return_true' ) ) {


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 · 2026-05-21T05:57:11Z

 	 * separate block styles, then abort.
 	 */
-	add_filter( 'should_load_separate_core_block_assets', '__return_true', 0 );
+	if ( ! has_filter( 'should_load_separate_core_block_assets', '__return_true' ) ) {


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 · 2026-05-21T05:57:18Z

 	 * has explicitly opted out of loading block styles on demand, then abort.
 	 */
-	add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
+	if ( ! has_filter( 'should_load_block_assets_on_demand', '__return_true' ) ) {


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 · 2026-05-21T05:57:27Z


 	// Add hooks which require the presence of the output buffer. Ideally the above two filters could be added here, but they run too early.
-	add_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' );
+	if ( ! has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ) ) {


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' ) ) {

…hooks. has_filter() and has_action() return the priority when a callback is registered, so priority 0 must be compared with false === instead of !. Adds regression tests for duplicate hook registration and for #64846 when WP_Styles is constructed before init. Props westonruter. Fixes #65272.

itzmekhokan · 2026-05-21T07:15:43Z

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.

itzmekhokan added 3 commits May 20, 2026 10:11

Tests: Add regression test for classic theme block theme styles on de…

0e370e3

…mand. Props Khokan Sardar. Fixes #65272.

Tests: Fix PHPCS class closing brace spacing for #65272 regression test.

1ee63f2

westonruter reviewed May 21, 2026

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Script Loader: Restore init hook so block theme styles load on demand in classic themes#11887

Script Loader: Restore init hook so block theme styles load on demand in classic themes#11887
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	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' ) ) {

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

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

	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' ) ) {

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