Ticket #40018: 40018.diff

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

@@ -324 +324 @@
                  */
                 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 ) ) {