[wp-trac] [WordPress Trac] #64846: Loading separate styles on demand thwarted by plugins that register styles at 'init' with priority less than 8

[WordPress Trac]

#64846: Loading separate styles on demand thwarted by plugins that register styles
at 'init' with priority less than 8
---------------------------+-------------------------
 Reporter:  westonruter    |      Owner:  westonruter
     Type:  defect (bug)   |     Status:  assigned
 Priority:  normal         |  Milestone:  7.0
Component:  Script Loader  |    Version:  6.9
 Severity:  normal         |   Keywords:
  Focuses:  performance    |
---------------------------+-------------------------
 This is a follow-up to #64099.

 With Elementor [https://github.com/elementor/elementor/pull/35084 patched]
 to re-enable loading block styles on demand, I was confused to find that
 the single block library stylesheet was being loaded in spite of the
 output buffer being started and other styles being hoisted as expected.
 With Elementor deactivated, I would see on the frontend as expected the
 non-combined block-library `common.css` loaded:

 {{{
 <style id="wp-block-library-inline-css">
 ...

 /*# sourceURL=/wp-includes/css/dist/block-library/common.css */
 </style>
 }}}

 But with Elementor active, I see:

 {{{
 <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' />
 }}}

 The issue is that the elementor plugin initializes at the `init` action
 with priority 0
 ([https://github.com/elementor/elementor/blob/2a6322e09517d6149523a1b1aac8df9be144669a/includes/plugin.php#L825
 source]) which causes the default styles to be registered earlier than
 normal. The `Elementor\Core\Page_Assets\Loader` component which does
 `wp_register_style()`:

 {{{
 #0 /var/www/src/wp-includes/script-loader.php(1829):
 WP_Dependencies->add()
 #1 /var/www/src/wp-includes/class-wp-hook.php(347): wp_default_styles()
 #2 /var/www/src/wp-includes/class-wp-hook.php(371):
 WP_Hook->apply_filters()
 #3 /var/www/src/wp-includes/plugin.php(570): WP_Hook->do_action()
 #4 /var/www/src/wp-includes/class-wp-styles.php(121):
 do_action_ref_array()
 #5 /var/www/src/wp-includes/functions.wp-styles.php(24):
 WP_Styles->__construct()
 #6 /var/www/src/wp-includes/functions.wp-styles.php(132): wp_styles()
 #7 /var/www/src/wp-content/plugins/elementor/core/page-
 assets/loader.php(172): wp_register_style()
 #8 /var/www/src/wp-content/plugins/elementor/core/page-
 assets/loader.php(181):
 Elementor\Core\Page_Assets\Loader->register_assets()
 #9 /var/www/src/wp-content/plugins/elementor/includes/plugin.php(711):
 Elementor\Core\Page_Assets\Loader->__construct()
 #10 /var/www/src/wp-content/plugins/elementor/includes/plugin.php(622):
 Elementor\Plugin->init_components()
 #11 /var/www/src/wp-includes/class-wp-hook.php(347):
 Elementor\Plugin->init()
 #12 /var/www/src/wp-includes/class-wp-hook.php(371):
 WP_Hook->apply_filters()
 #13 /var/www/src/wp-includes/plugin.php(522): WP_Hook->do_action()
 }}}

 At this point, when `wp_default_styles()` is called,
 [https://github.com/WordPress/wordpress-
 develop/blob/2bb252ae83bc78852c0d2e11749d640ac8b6bc8a/src/wp-includes
 /script-loader.php#L1819-L1823 this logic] is run to register the `wp-
 block-library` style:

 {{{#!php
 <?php
 $path   = "/wp-includes/css/dist/$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 issue is that `wp_should_load_separate_core_block_assets()` here
 returns `false` because `wp_load_classic_theme_block_styles_on_demand()`
 has run yet, since it runs later in the `init` action
 [https://github.com/WordPress/wordpress-
 develop/blob/2bb252ae83bc78852c0d2e11749d640ac8b6bc8a/src/wp-includes
 /default-filters.php#L610 at priority 8]:

 {{{
 add_action( 'init', 'wp_load_classic_theme_block_styles_on_demand', 8 );
 // Must happen before register_core_block_style_handles() at priority 9.
 }}}

 This function is responsible for adding the adding the filters to opt-in
 to loading separate block styles on demand:

 {{{
 add_filter( 'should_load_separate_core_block_assets', '__return_true', 0
 );
 add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
 }}}

 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.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/64846>
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