Customize: Fix selective refresh when customizing the 404 template.

Overrides the 404 status during partial refresh requests to serve back 200 so that the request is not deemed a failure and invoke the fallback behavior (full refresh).

See #27355.
Fixes #40018.

Built from https://develop.svn.wordpress.org/trunk@40316


git-svn-id: http://core.svn.wordpress.org/trunk@40223 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/customize/class-wp-customize-selective-refresh.php

@@ -324,10 +324,13 @@ final class WP_Customize_Selective_Refresh {
 		 */
 		if ( ! is_customize_preview() ) {
 			wp_send_json_error( 'expected_customize_preview', 403 );
-		} else if ( ! isset( $_POST['partials'] ) ) {
+		} elseif ( ! isset( $_POST['partials'] ) ) {
 			wp_send_json_error( 'missing_partials', 400 );
 		}
 
+		// Ensure that doing selective refresh on 404 template doesn't result in fallback rendering behavior (full refreshes).
+		status_header( 200 );
+
 		$partials = json_decode( wp_unslash( $_POST['partials'] ), true );
 
 		if ( ! is_array( $partials ) ) {

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-40315';
+$wp_version = '4.8-alpha-40316';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.