FSE: Add basic support for child themes (#34354)

Author: Mamaduka

lib/class-wp-theme-json-resolver-gutenberg.php

@@ -493,16 +493,8 @@ public static function theme_has_support() {
 	 * @return string The whole file path or empty if the file doesn't exist.
 	 */
 	private static function get_file_path_from_theme( $file_name ) {
-		// This used to be a locate_template call.
-		// However, that method proved problematic
-		// due to its use of constants (STYLESHEETPATH)
-		// that threw errors in some scenarios.
-		//
-		// When the theme.json merge algorithm properly supports
-		// child themes, this should also fallback
-		// to the template path, as locate_template did.
 		$located   = '';
-		$candidate = get_stylesheet_directory() . '/' . $file_name;
+		$candidate = get_theme_file_path( $file_name );
 		if ( is_readable( $candidate ) ) {
 			$located = $candidate;
 		}

lib/full-site-editing/full-site-editing.php

@@ -11,7 +11,7 @@
  * @return boolean Whether the current theme is an FSE theme or not.
  */
 function gutenberg_is_fse_theme() {
-	return is_readable( get_stylesheet_directory() . '/block-templates/index.html' );
+	return is_readable( get_theme_file_path( '/block-templates/index.html' ) );
 }
 
 /**

packages/block-library/src/template-part/index.php

@@ -53,7 +53,7 @@ function render_block_core_template_part( $attributes ) {
 		} else {
 			// Else, if the template part was provided by the active theme,
 			// render the corresponding file content.
-			$template_part_file_path = get_stylesheet_directory() . '/block-template-parts/' . $attributes['slug'] . '.html';
+			$template_part_file_path = get_theme_file_path( '/block-template-parts/' . $attributes['slug'] . '.html' );
 			if ( 0 === validate_file( $attributes['slug'] ) && file_exists( $template_part_file_path ) ) {
 				$content = _gutenberg_inject_theme_attribute_in_content( file_get_contents( $template_part_file_path ) );
 			}