diff --git a/src/wp-content/themes/twentyfifteen/js/functions.js b/src/wp-content/themes/twentyfifteen/js/functions.js
index 64a696860d..9adad87e39 100644
--- a/src/wp-content/themes/twentyfifteen/js/functions.js
+++ b/src/wp-content/themes/twentyfifteen/js/functions.js
@@ -11,16 +11,15 @@
 	    topOffset = 0, bodyHeight, sidebarHeight, resizeTimer,
 	    secondary, button;
 
-
-	function initMainNavigation() {
+	function initMainNavigation( container ) {
 		// Add dropdown toggle that display child menu items.
-		$( '.main-navigation .menu-item-has-children > a' ).after( '<button class="dropdown-toggle" aria-expanded="false">' + screenReaderText.expand + '</button>' );
+		container.find( '.menu-item-has-children > a' ).after( '<button class="dropdown-toggle" aria-expanded="false">' + screenReaderText.expand + '</button>' );
 
 		// Toggle buttons and submenu items with active children menu items.
-		$( '.main-navigation .current-menu-ancestor > button' ).addClass( 'toggle-on' );
-		$( '.main-navigation .current-menu-ancestor > .sub-menu' ).addClass( 'toggled-on' );
+		container.find( '.current-menu-ancestor > button' ).addClass( 'toggle-on' );
+		container.find( '.current-menu-ancestor > .sub-menu' ).addClass( 'toggled-on' );
 
-		$( '.dropdown-toggle' ).click( function( e ) {
+		container.find( '.dropdown-toggle' ).click( function( e ) {
 			var _this = $( this );
 			e.preventDefault();
 			_this.toggleClass( 'toggle-on' );
@@ -29,8 +28,20 @@
 			_this.html( _this.html() === screenReaderText.expand ? screenReaderText.collapse : screenReaderText.expand );
 		} );
 	}
-	initMainNavigation();
-	$( document ).on( 'customize-preview-menu-refreshed', initMainNavigation );
+	initMainNavigation( $( '.main-navigation' ) );
+
+	// Re-initialize the main navigation when it is updated, persisting any existing submenu expanded states.
+	$( document ).on( 'customize-preview-menu-refreshed', function( e, params ) {
+		if ( 'primary' === params.wpNavMenuArgs.theme_location ) {
+			initMainNavigation( params.newContainer );
+
+			// Re-sync expanded states from oldContainer.
+			params.oldContainer.find( '.dropdown-toggle.toggle-on' ).each(function() {
+				var containerId = $( this ).parent().prop( 'id' );
+				$( params.newContainer ).find( '#' + containerId + ' > .dropdown-toggle' ).triggerHandler( 'click' );
+			});
+		}
+	});
 
 	secondary = $( '#secondary' );
 	button = $( '.site-branding' ).find( '.secondary-toggle' );
diff --git a/src/wp-includes/js/customize-preview-nav-menus.js b/src/wp-includes/js/customize-preview-nav-menus.js
index 017d1f1216..814c6f21d7 100644
--- a/src/wp-includes/js/customize-preview-nav-menus.js
+++ b/src/wp-includes/js/customize-preview-nav-menus.js
@@ -174,7 +174,7 @@
 		 * @param {int} instanceNumber
 		 */
 		refreshMenuInstance : function( instanceNumber ) {
-			var data, menuId, customized, container, request, wpNavArgs, instance, containerInstanceClassName;
+			var data, menuId, customized, container, request, wpNavMenuArgs, instance, containerInstanceClassName;
 
 			if ( ! settings.navMenuInstanceArgs[ instanceNumber ] ) {
 				throw new Error( 'unknown_instance_number' );
@@ -227,10 +227,10 @@
 			data.customized = JSON.stringify( customized );
 			data[ settings.renderNoncePostKey ] = settings.renderNonceValue;
 
-			wpNavArgs = $.extend( {}, instance );
-			data.wp_nav_menu_args_hash = wpNavArgs.args_hash;
-			delete wpNavArgs.args_hash;
-			data.wp_nav_menu_args = JSON.stringify( wpNavArgs );
+			wpNavMenuArgs = $.extend( {}, instance );
+			data.wp_nav_menu_args_hash = wpNavMenuArgs.args_hash;
+			delete wpNavMenuArgs.args_hash;
+			data.wp_nav_menu_args = JSON.stringify( wpNavMenuArgs );
 
 			container.addClass( 'customize-partial-refreshing' );
 
@@ -252,7 +252,8 @@
 				previousContainer.replaceWith( container );
 				eventParam = {
 					instanceNumber: instanceNumber,
-					wpNavArgs: wpNavArgs,
+					wpNavArgs: wpNavMenuArgs, // @deprecated
+					wpNavMenuArgs: wpNavMenuArgs,
 					oldContainer: previousContainer,
 					newContainer: container
 				};