Classic theme on demand block styles loading regression

[rgcodes]

Issue: WordPress 7.0 breaks native block styles loading for classic themes.

Steps to reproduce: A bare classic theme (style.css, functions.php with wp-block-styles theme support, index.php). Zero plugins installed. Add a group block with a background to a page in the editor.

Alternating between WordPress 6.9.4 and 7.0 reliably removes/adds the bug confirming the bug exists in 7.0

Symptom: Group block has default padding in the editor but no padding on the front-end. Native block styles are missing on the front-end

Diagnosis:
The hook registration for wp_load_classic_theme_block_styles_on_demand() — which adds the should_load_separate_core_block_assets filter — changed in wp-includes/default-filters.php:

<?php
// 6.9
add_action( 'init', 'wp_load_classic_theme_block_styles_on_demand', 8 );

// 7.0 (default-filters.php:609)
add_action( 'wp_default_styles', 'wp_load_classic_theme_block_styles_on_demand', 0 );

register_core_block_style_handles() (wp-includes/blocks/index.php:29, hooked at init:9 on line 144) has an early return on line 32:

<?php
if ( ! wp_should_load_separate_core_block_assets() ) {
    return;
}

wp_default_styles does not fire before init:9 in Core.

At init:9 (where register_core_block_style_handles() runs) → wp_should_load_separate_core_block_assets() returns false (its filter has no callbacks yet) → per-block wp-block-{name}-theme handles never registered with URLs.

At wp_enqueue_scripts (where wp_common_block_scripts_and_styles() runs at script-loader.php:2502) → wp_should_load_separate_core_block_assets() returns true (something has since instantiated wp_styles(), firing the filter attachment) → the ! wp_should_load_separate_core_block_assets() guard skips enqueuing wp-block-library-theme (the combined stylesheet) too.

Resulting in no block styles being loaded at all

Workaround:
Adding the following to a theme or plugin fixes the assets but shouldn't be needed:

<?php
add_filter( 'should_load_separate_core_block_assets', '__return_true' );
add_filter( 'should_load_block_assets_on_demand', '__return_true' );

[sabernhardt]

Hi and thanks for the report!

This is already tracked on #65272, if you could continue the conversation there.