Ticket #43113: 43113.4.diff

src/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

@@ -605 +605 @@
                         $item->post_title = $item->title;
                 }
 
+                // 'classes' should be an array, as in wp_setup_nav_menu_item().
+                if ( isset( $item->classes ) && is_scalar( $item->classes ) ) {
+                        $item->classes = explode( ' ', $item->classes );
+                }
+
                 $item->ID    = $this->post_id;
                 $item->db_id = $this->post_id;
                 $post        = new WP_Post( (object) $item );

tests/phpunit/tests/customize/nav-menu-item-setting.php

@@ -906 +906 @@
                         'target'           => '',
                         'attr_title'       => '">att \o/ o\'o empted <b>baddie</b>',
                         'description'      => 'Attempted \o/ o\'o <b>markup</b>',
-                        'classes'          => '',
+                        'classes'          => 'class-1 class-2',
                         'xfn'              => '',
                         'status'           => 'publish',
                         'original_title'   => '',
@@ -940 +940 @@
                 $expected = apply_filters( 'nav_menu_attr_title', wp_unslash( apply_filters( 'excerpt_save_pre', wp_slash( $post_value['attr_title'] ) ) ) );
                 $this->assertSame( $expected, $nav_menu_item->attr_title );
                 $this->assertSame( 'Attempted \o/ o&#8217;o markup', $nav_menu_item->description );
+                $this->assertSame( array( 'class-1', 'class-2' ), $nav_menu_item->classes );
         }
 
         /**