Ticket #39103: 39103.1.dff

diff --git src/wp-admin/js/customize-nav-menus.js src/wp-admin/js/customize-nav-menus.js
index b7556f6..a37d2c4 100644
--- src/wp-admin/js/customize-nav-menus.js
+++ src/wp-admin/js/customize-nav-menus.js
@@ -1169,7 +1169,11 @@
 
                        // @todo It would be better if this was added directly on the setting itself, as opposed to the control.
                        control.setting.validate = function( value ) {
-                               return parseInt( value, 10 );
+                               if ( '' === value ) {
+                                       return 0;
+                               } else {
+                                       return parseInt( value, 10 );
+                               }
                        };
 
                        // Edit menu button.
diff --git src/wp-includes/class-wp-customize-manager.php src/wp-includes/class-wp-customize-manager.php
index 127d572..2bb291f 100644
--- src/wp-includes/class-wp-customize-manager.php
+++ src/wp-includes/class-wp-customize-manager.php
@@ -227,6 +227,13 @@ final class WP_Customize_Manager {
        private $_changeset_data;
 
        /**
+        * Whether the instance will only be used for saving, and not previewing.
+        *
+        * @var
+        */
+       public $saving = false;
+
+       /**
         * Constructor.
         *
         * @since 3.4.0
@@ -243,7 +250,7 @@ final class WP_Customize_Manager {
        public function __construct( $args = array() ) {
 
                $args = array_merge(
-                       array_fill_keys( array( 'changeset_uuid', 'theme', 'messenger_channel' ), null ),
+                       array_fill_keys( array( 'changeset_uuid', 'theme', 'messenger_channel', 'saving' ), null ),
                        $args
                );
 
@@ -268,6 +275,7 @@ final class WP_Customize_Manager {
                $this->theme = wp_get_theme( $args['theme'] );
                $this->messenger_channel = $args['messenger_channel'];
                $this->_changeset_uuid = $args['changeset_uuid'];
+               $this->saving = ! empty( $args['saving'] ) || $this->doing_ajax( 'customize_save' );
 
                require_once( ABSPATH . WPINC . '/class-wp-customize-setting.php' );
                require_once( ABSPATH . WPINC . '/class-wp-customize-panel.php' );
@@ -741,7 +749,7 @@ final class WP_Customize_Manager {
                 * may short-circuit because it will detect that there are no changes to
                 * make.
                 */
-               if ( ! $this->doing_ajax( 'customize_save' ) ) {
+               if ( ! $this->saving ) {
                        foreach ( $this->settings as $setting ) {
                                $setting->preview();
                        }
diff --git src/wp-includes/class-wp-customize-nav-menus.php src/wp-includes/class-wp-customize-nav-menus.php
index ca1b72f..1bffeb0 100644
--- src/wp-includes/class-wp-customize-nav-menus.php
+++ src/wp-includes/class-wp-customize-nav-menus.php
@@ -542,10 +542,12 @@ final class WP_Customize_Nav_Menus {
                        }
                }
                $this->manager->add_dynamic_settings( $nav_menus_setting_ids );
-               foreach ( $nav_menus_setting_ids as $setting_id ) {
-                       $setting = $this->manager->get_setting( $setting_id );
-                       if ( $setting ) {
-                               $setting->preview();
+               if ( ! $this->manager->saving ) {
+                       foreach ( $nav_menus_setting_ids as $setting_id ) {
+                               $setting = $this->manager->get_setting( $setting_id );
+                               if ( $setting ) {
+                                       $setting->preview();
+                               }
                        }
                }
 
diff --git src/wp-includes/class-wp-customize-widgets.php src/wp-includes/class-wp-customize-widgets.php
index 7403119..7fa38e6 100644
--- src/wp-includes/class-wp-customize-widgets.php
+++ src/wp-includes/class-wp-customize-widgets.php
@@ -209,10 +209,10 @@ final class WP_Customize_Widgets {
 
                /*
                 * Preview settings right away so that widgets and sidebars will get registered properly.
-                * But don't do this if a customize_save because this will cause WP to think there is nothing
-                * changed that needs to be saved.
+                * But don't do this when saving the changeset because this will cause WP to think there
+                * is nothing changed that needs to be saved.
                 */
-               if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
+               if ( ! $this->manager->saving ) {
                        foreach ( $settings as $setting ) {
                                $setting->preview();
                        }
@@ -509,7 +509,7 @@ final class WP_Customize_Widgets {
                        }
                }
 
-               if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
+               if ( ! $this->manager->saving ) {
                        foreach ( $new_setting_ids as $new_setting_id ) {
                                $this->manager->get_setting( $new_setting_id )->preview();
                        }