Disable client-side media processing in Playground

lib/compat/wordpress-7.0/media.php

@@ -16,6 +16,13 @@
  * @return bool Whether client-side media processing is enabled.
  */
 function gutenberg_is_client_side_media_processing_enabled() {
+	// Disable in php-wasm environments (e.g. WordPress Playground) where
+	// cross-origin isolation headers conflict with the iframe architecture.
+	// See https://github.com/WordPress/gutenberg/issues/75941
+	if ( 'wasm' === PHP_SAPI ) {
+		return false;
+	}
+
 	/**
 	 * Filters whether client-side media processing is enabled.
 	 *

phpunit/media/media-processing-test.php

@@ -210,6 +210,24 @@ public function test_client_side_media_processing_enabled_by_default_in_plugin()
 		$this->assertTrue( gutenberg_is_client_side_media_processing_enabled() );
 	}
 
+	/**
+	 * Tests that client-side media processing is disabled in php-wasm environments.
+	 *
+	 * Note: PHP_SAPI is a runtime constant that cannot be changed in tests.
+	 * This test verifies the check exists in the function source code.
+	 * When PHP_SAPI === 'wasm' (WordPress Playground), the function returns
+	 * false before the filter is ever applied, preventing cross-origin
+	 * isolation headers from conflicting with Playground's iframe architecture.
+	 *
+	 * @covers ::gutenberg_is_client_side_media_processing_enabled
+	 */
+	public function test_client_side_media_processing_disabled_in_wasm_environments() {
+		// Verify the function contains the wasm SAPI check.
+		$function = new ReflectionFunction( 'gutenberg_is_client_side_media_processing_enabled' );
+		$source   = file_get_contents( $function->getFileName() );
+		$this->assertStringContainsString( "'wasm' === PHP_SAPI", $source );
+	}
+
 	/**
 	 * Tests that client-side media processing can be disabled via filter.
 	 *