Avoid enabling separate/on-demand block styles if theme does not support wp-block-styles

Author: westonruter

src/wp-includes/script-loader.php

@@ -3593,13 +3593,21 @@ function wp_load_classic_theme_block_styles_on_demand() {
 	}
 
 	/*
-	 * Load separate block styles so that the large block-library stylesheet is not enqueued unconditionally,
-	 * and so that block-specific styles will only be enqueued when they are used on the page.
+	 * If the theme supports block styles, add filters to ensure they are loaded separately and on demand. Without this,
+	 * if a theme does not want or support block styles, then enabling these filters can result in undesired separate
+	 * block-specific styles being enqueued, though a theme may also be trying to nullify the wp-block-library
+	 * stylesheet.
 	 */
-	add_filter( 'should_load_separate_core_block_assets', '__return_true', 0 );
+	if ( current_theme_supports( 'wp-block-styles' ) ) {
+		/*
+		 * Load separate block styles so that the large block-library stylesheet is not enqueued unconditionally,
+		 * and so that block-specific styles will only be enqueued when they are used on the page.
+		 */
+		add_filter( 'should_load_separate_core_block_assets', '__return_true', 0 );
 
-	// Also ensure that block assets are loaded on demand (although the default value is from should_load_separate_core_block_assets).
-	add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
+		// Also ensure that block assets are loaded on demand (although the default value is from should_load_separate_core_block_assets).
+		add_filter( 'should_load_block_assets_on_demand', '__return_true', 0 );
+	}
 
 	// 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' );

tests/phpunit/tests/template.php

@@ -78,6 +78,11 @@ public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
 	 */
 	protected $original_wp_styles;
 
+	/**
+	 * @var array|null
+	 */
+	protected $original_theme_features;
+
 	public function set_up() {
 		parent::set_up();
 		$this->original_default_mimetype = ini_get( 'default_mimetype' );
@@ -110,13 +115,17 @@ public function set_up() {
 		$wp_styles                 = null;
 		wp_scripts();
 		wp_styles();
+
+		$this->original_theme_features = $GLOBALS['_wp_theme_features'];
 	}
 
 	public function tear_down() {
 		global $wp_scripts, $wp_styles;
 		$wp_scripts = $this->original_wp_scripts;
 		$wp_styles  = $this->original_wp_styles;
 
+		$GLOBALS['_wp_theme_features'] = $this->original_theme_features;
+
 		ini_set( 'default_mimetype', $this->original_default_mimetype );
 		unregister_post_type( 'cpt' );
 		unregister_taxonomy( 'taxo' );
@@ -930,12 +939,35 @@ public function test_wp_load_classic_theme_block_styles_on_demand_in_classic_the
 	}
 
 	/**
-	 * Tests that wp_load_classic_theme_block_styles_on_demand() adds the expected hooks for classic themes.
+	 * Tests that wp_load_classic_theme_block_styles_on_demand() adds the expected hooks for classic theme which supports block styles.
+	 *
+	 * @ticket 64099
+	 * @ticket 64150
+	 * @covers ::wp_load_classic_theme_block_styles_on_demand
+	 */
+	public function test_wp_load_classic_theme_block_styles_on_demand_in_classic_theme_with_block_styles_support(): void {
+		switch_theme( 'default' );
+		add_theme_support( 'wp-block-styles' );
+
+		$this->assertFalse( wp_should_load_separate_core_block_assets(), 'Expected wp_should_load_separate_core_block_assets() to return false initially.' );
+		$this->assertFalse( wp_should_load_block_assets_on_demand(), 'Expected wp_should_load_block_assets_on_demand() to return true' );
+		$this->assertFalse( has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ), 'Expected wp_template_enhancement_output_buffer_started action to be added for classic themes.' );
+
+		wp_load_classic_theme_block_styles_on_demand();
+
+		$this->assertTrue( wp_should_load_separate_core_block_assets(), 'Expected wp_should_load_separate_core_block_assets() filters to return true because the theme does support wp-block-styles' );
+		$this->assertTrue( wp_should_load_block_assets_on_demand(), 'Expected wp_should_load_block_assets_on_demand() to return true because the theme does support wp-block-styles' );
+		$this->assertNotFalse( has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ), 'Expected wp_template_enhancement_output_buffer_started action to be added for classic themes.' );
+	}
+
+	/**
+	 * Tests that wp_load_classic_theme_block_styles_on_demand() adds the expected hooks for classic theme which does not support block styles.
 	 *
 	 * @ticket 64099
+	 * @ticket 64150
 	 * @covers ::wp_load_classic_theme_block_styles_on_demand
 	 */
-	public function test_wp_load_classic_theme_block_styles_on_demand_in_classic_theme(): void {
+	public function test_wp_load_classic_theme_block_styles_on_demand_in_classic_theme_without_block_styles_support(): void {
 		switch_theme( 'default' );
 
 		$this->assertFalse( wp_should_load_separate_core_block_assets(), 'Expected wp_should_load_separate_core_block_assets() to return false initially.' );
@@ -944,8 +976,8 @@ public function test_wp_load_classic_theme_block_styles_on_demand_in_classic_the
 
 		wp_load_classic_theme_block_styles_on_demand();
 
-		$this->assertTrue( wp_should_load_separate_core_block_assets(), 'Expected wp_should_load_separate_core_block_assets() filters to return true' );
-		$this->assertTrue( wp_should_load_block_assets_on_demand(), 'Expected wp_should_load_block_assets_on_demand() to return true' );
+		$this->assertFalse( wp_should_load_separate_core_block_assets(), 'Expected wp_should_load_separate_core_block_assets() filters to return false because the theme does not support wp-block-styles' );
+		$this->assertFalse( wp_should_load_block_assets_on_demand(), 'Expected wp_should_load_block_assets_on_demand() to return false because the theme does not support wp-block-styles' );
 		$this->assertNotFalse( has_action( 'wp_template_enhancement_output_buffer_started', 'wp_hoist_late_printed_styles' ), 'Expected wp_template_enhancement_output_buffer_started action to be added for classic themes.' );
 	}