Script Loader: Move wp_load_classic_theme_block_styles_on_demand() from init to wp_default_styles.

This ensures the filters to opt in to loading separate block styles on demand are added at the moment `WP_Styles` is constructed. This accounts for styles being registered at the `init` action before `register_core_block_style_handles()` runs at priority 9. Without this, the `wp-block-library` stylesheet may get registered with the full combined block styles as `style.css` instead of just `common.css`, due to `wp_should_load_block_assets_on_demand()` still returning false. The `wp_default_styles` action still runs during `init`.

Developed in #11232

Follow-up to r61008.

Props westonruter, adamsilverstein.
See #64099.
Fixes #64846.


git-svn-id: https://develop.svn.wordpress.org/trunk@61981 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/default-filters.php

@@ -607,7 +607,7 @@
 add_action( 'enqueue_block_assets', 'wp_enqueue_classic_theme_styles' );
 add_action( 'enqueue_block_assets', 'wp_enqueue_registered_block_scripts_and_styles' );
 add_action( 'enqueue_block_assets', 'enqueue_block_styles_assets', 30 );
-add_action( 'init', 'wp_load_classic_theme_block_styles_on_demand', 8 ); // Must happen before register_core_block_style_handles() at priority 9.
+add_action( 'wp_default_styles', 'wp_load_classic_theme_block_styles_on_demand', 0 ); // Must happen before wp_default_styles() and register_core_block_style_handles().
 /*
  * `wp_enqueue_registered_block_scripts_and_styles` is bound to both
  * `enqueue_block_editor_assets` and `enqueue_block_assets` hooks

src/wp-includes/script-loader.php

@@ -3674,11 +3674,15 @@ function wp_remove_surrounding_empty_script_tags( $contents ) {
 /**
  * Adds hooks to load block styles on demand in classic themes.
  *
+ * This function must be called before {@see wp_default_styles()} and {@see register_core_block_style_handles()} so that
+ * the filters are added to cause {@see wp_should_load_separate_core_block_assets()} to return true.
+ *
  * @since 6.9.0
+ * @since 7.0.0 This is now invoked at the `wp_default_styles` action with priority 0 instead of at `init` with priority 8.
  *
  * @see _add_default_theme_supports()
  */
-function wp_load_classic_theme_block_styles_on_demand() {
+function wp_load_classic_theme_block_styles_on_demand(): void {
 	// This is not relevant to block themes, as they are opted in to loading separate styles on demand via _add_default_theme_supports().
 	if ( wp_is_block_theme() ) {
 		return;

tests/phpunit/tests/template.php

@@ -1963,6 +1963,10 @@ public function test_wp_hoist_late_printed_styles( ?Closure $set_up, string $con
 		// `_print_emoji_detection_script()` assumes `wp-includes/js/wp-emoji-loader.js` is present:
 		self::touch( ABSPATH . WPINC . '/js/wp-emoji-loader.js' );
 
+		if ( $set_up ) {
+			$set_up();
+		}
+
 		switch_theme( 'default' );
 		global $wp_styles;
 		$wp_styles = null;
@@ -2001,10 +2005,6 @@ static function () {
 			}
 		);
 
-		if ( $set_up ) {
-			$set_up();
-		}
-
 		wp_load_classic_theme_block_styles_on_demand();
 
 		// Ensure that separate core block assets get registered.