diff --git a/wp-includes/class-wp-customize-manager.php b/wp-includes/class-wp-customize-manager.php
index b26833beb7..9ba93c575c 100644
--- a/wp-includes/class-wp-customize-manager.php
+++ b/wp-includes/class-wp-customize-manager.php
@@ -173,6 +173,14 @@ final class WP_Customize_Manager {
 	 */
 	protected $messenger_channel;
 
+	/**
+	 * Whether settings should be previewed.
+	 *
+	 * @since 4.9.0
+	 * @var bool
+	 */
+	protected $settings_previewed;
+
 	/**
 	 * Unsanitized values for Customize Settings parsed from $_POST['customized'].
 	 *
@@ -213,15 +221,16 @@ final class WP_Customize_Manager {
 	 * @param array $args {
 	 *     Args.
 	 *
-	 *     @type string $changeset_uuid    Changeset UUID, the post_name for the customize_changeset post containing the customized state. Defaults to new UUID.
-	 *     @type string $theme             Theme to be previewed (for theme switch). Defaults to customize_theme or theme query params.
-	 *     @type string $messenger_channel Messenger channel. Defaults to customize_messenger_channel query param.
+	 *     @type string $changeset_uuid     Changeset UUID, the post_name for the customize_changeset post containing the customized state. Defaults to new UUID.
+	 *     @type string $theme              Theme to be previewed (for theme switch). Defaults to customize_theme or theme query params.
+	 *     @type string $messenger_channel  Messenger channel. Defaults to customize_messenger_channel query param.
+	 *     @type bool   $settings_previewed If settings should be previewed. Defaults to true.
 	 * }
 	 */
 	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', 'settings_previewed' ), null ),
 			$args
 		);
 
@@ -242,9 +251,14 @@ final class WP_Customize_Manager {
 			$args['messenger_channel'] = sanitize_key( wp_unslash( $_REQUEST['customize_messenger_channel'] ) );
 		}
 
+		if ( ! isset( $args['settings_previewed'] ) ) {
+			$args['settings_previewed'] = true;
+		}
+
 		$this->original_stylesheet = get_stylesheet();
 		$this->theme = wp_get_theme( $args['theme'] );
 		$this->messenger_channel = $args['messenger_channel'];
+		$this->settings_previewed = ! empty( $args['settings_previewed'] );
 		$this->_changeset_uuid = $args['changeset_uuid'];
 
 		require_once( ABSPATH . WPINC . '/class-wp-customize-setting.php' );
@@ -621,6 +635,18 @@ final class WP_Customize_Manager {
 		do_action( 'stop_previewing_theme', $this );
 	}
 
+	/**
+	 * Gets whether settings are or will be previewed.
+	 *
+	 * @since 4.9.0
+	 * @see WP_Customize_Setting::preview()
+	 *
+	 * @return bool
+	 */
+	public function settings_previewed() {
+		return $this->settings_previewed;
+	}
+
 	/**
 	 * Get the changeset UUID.
 	 *
@@ -728,15 +754,7 @@ final class WP_Customize_Manager {
 		 */
 		do_action( 'customize_register', $this );
 
-		/*
-		 * Note that settings must be previewed here even outside the customizer preview
-		 * and also in the customizer pane itself. This is to enable loading an existing
-		 * changeset into the customizer. Previewing the settings only has to be prevented
-		 * in the case of a customize_save action because then update_option()
-		 * may short-circuit because it will detect that there are no changes to
-		 * make.
-		 */
-		if ( ! $this->doing_ajax( 'customize_save' ) ) {
+		if ( $this->settings_previewed() ) {
 			foreach ( $this->settings as $setting ) {
 				$setting->preview();
 			}
diff --git a/wp-includes/class-wp-customize-nav-menus.php b/wp-includes/class-wp-customize-nav-menus.php
index 9b24fa0b67..3d7b1f0deb 100644
--- a/wp-includes/class-wp-customize-nav-menus.php
+++ b/wp-includes/class-wp-customize-nav-menus.php
@@ -526,13 +526,10 @@ final class WP_Customize_Nav_Menus {
 				$nav_menus_setting_ids[] = $setting_id;
 			}
 		}
-		$this->manager->add_dynamic_settings( $nav_menus_setting_ids );
-		if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
-			foreach ( $nav_menus_setting_ids as $setting_id ) {
-				$setting = $this->manager->get_setting( $setting_id );
-				if ( $setting ) {
-					$setting->preview();
-				}
+		$settings = $this->manager->add_dynamic_settings( $nav_menus_setting_ids );
+		if ( $this->manager->settings_previewed() ) {
+			foreach ( $settings as $setting ) {
+				$setting->preview();
 			}
 		}
 
diff --git a/wp-includes/class-wp-customize-widgets.php b/wp-includes/class-wp-customize-widgets.php
index 856a832ed9..973411b520 100644
--- a/wp-includes/class-wp-customize-widgets.php
+++ b/wp-includes/class-wp-customize-widgets.php
@@ -211,12 +211,7 @@ final class WP_Customize_Widgets {
 
 		$settings = $this->manager->add_dynamic_settings( array_unique( $widget_setting_ids ) );
 
-		/*
-		 * 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.
-		 */
-		if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
+		if ( $this->manager->settings_previewed() ) {
 			foreach ( $settings as $setting ) {
 				$setting->preview();
 			}
@@ -506,7 +501,7 @@ final class WP_Customize_Widgets {
 			}
 		}
 
-		if ( ! $this->manager->doing_ajax( 'customize_save' ) ) {
+		if ( $this->manager->settings_previewed() ) {
 			foreach ( $new_setting_ids as $new_setting_id ) {
 				$this->manager->get_setting( $new_setting_id )->preview();
 			}
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 33b491ab1c..a1cc0866b2 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -2816,8 +2816,24 @@ function _wp_customize_include() {
 		$messenger_channel = sanitize_key( $input_vars['customize_messenger_channel'] );
 	}
 
+	/*
+	 * Note that settings must be previewed even outside the customizer preview
+	 * and also in the customizer pane itself. This is to enable loading an existing
+	 * changeset into the customizer. Previewing the settings only has to be prevented
+	 * here in the case of a customize_save action because this will cause WP to think
+	 * there is nothing changed that needs to be saved.
+	 */
+	$is_customize_save_action = (
+		wp_doing_ajax()
+		&&
+		isset( $_REQUEST['action'] )
+		&&
+		'customize_save' === wp_unslash( $_REQUEST['action'] )
+	);
+	$settings_previewed = ! $is_customize_save_action;
+
 	require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
-	$GLOBALS['wp_customize'] = new WP_Customize_Manager( compact( 'changeset_uuid', 'theme', 'messenger_channel' ) );
+	$GLOBALS['wp_customize'] = new WP_Customize_Manager( compact( 'changeset_uuid', 'theme', 'messenger_channel', 'settings_previewed' ) );
 }
 
 /**
@@ -2849,7 +2865,10 @@ function _wp_customize_publish_changeset( $new_status, $old_status, $changeset_p
 
 	if ( empty( $wp_customize ) ) {
 		require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
-		$wp_customize = new WP_Customize_Manager( array( 'changeset_uuid' => $changeset_post->post_name ) );
+		$wp_customize = new WP_Customize_Manager( array(
+			'changeset_uuid' => $changeset_post->post_name,
+			'settings_previewed' => false,
+		) );
 	}
 
 	if ( ! did_action( 'customize_register' ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index e0a0135d65..60f63d46a4 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41204';
+$wp_version = '4.9-alpha-41205';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.