diff --git a/wp-content/themes/twentyfifteen/js/functions.js b/wp-content/themes/twentyfifteen/js/functions.js
index 64a696860d..9adad87e39 100644
--- a/wp-content/themes/twentyfifteen/js/functions.js
+++ b/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/wp-includes/js/customize-preview-nav-menus.js b/wp-includes/js/customize-preview-nav-menus.js
index 017d1f1216..814c6f21d7 100644
--- a/wp-includes/js/customize-preview-nav-menus.js
+++ b/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
 				};
diff --git a/wp-includes/js/customize-preview-nav-menus.min.js b/wp-includes/js/customize-preview-nav-menus.min.js
index e369907068..8e95eed516 100644
--- a/wp-includes/js/customize-preview-nav-menus.min.js
+++ b/wp-includes/js/customize-preview-nav-menus.min.js
@@ -1 +1 @@
-!function(a,b,c){"use strict";if(c&&c.customize){var d=c.customize,e={},f=200,g={},h={renderQueryVar:null,renderNonceValue:null,renderNoncePostKey:null,previewCustomizeNonce:null,requestUri:"/",theme:{active:!1,stylesheet:""},navMenuInstanceArgs:{}};d.MenusCustomizerPreview={init:function(){var a=this,c={};g=b.extend({},h),"undefined"!=typeof _wpCustomizePreviewNavMenusExports&&b.extend(g,_wpCustomizePreviewNavMenusExports),d.each(function(b,d){b.id=d,c[b.id]=!0,a.bindListener(b)}),d.preview.bind("setting",function(b){var e,f,g;b=b.slice(),e=b.shift(),f=b.shift(),g=d(e),g||(g=d.create(e,f)),g.id||(g.id=e),c[g.id]||(c[g.id]=!0,a.bindListener(g)&&g.callbacks.fireWith(g,[g(),null]))})},bindListener:function(a){var c,d;return(c=a.id.match(/^nav_menu\[(-?\d+)]$/))?(a.navMenuId=parseInt(c[1],10),a.bind(this.onChangeNavMenuSetting),!0):(c=a.id.match(/^nav_menu_item\[(-?\d+)]$/))?(a.navMenuItemId=parseInt(c[1],10),a.bind(this.onChangeNavMenuItemSetting),!0):(c=a.id.match(/^nav_menu_locations\[(.+?)]/),c?(d=c[1],a.bind(b.bind(function(){this.refreshMenuLocation(d)},this)),!0):!1)},onChangeNavMenuSetting:function(){var a=this;if(!a.navMenuId)throw new Error("Expected navMenuId property to be set.");d.MenusCustomizerPreview.refreshMenu(a.navMenuId)},onChangeNavMenuItemSetting:function(a,b){!b||!b.nav_menu_term_id||a&&b.nav_menu_term_id===a.nav_menu_term_id||d.MenusCustomizerPreview.refreshMenu(b.nav_menu_term_id),a&&a.nav_menu_term_id&&d.MenusCustomizerPreview.refreshMenu(a.nav_menu_term_id)},refreshMenu:function(a){var c=[];d.each(function(b,d){var e=d.match(/^nav_menu_locations\[(.+?)]/);e&&a===b()&&c.push(e[1])}),b.each(g.navMenuInstanceArgs,function(d,e){(a===d.menu||-1!==b.indexOf(c,d.theme_location))&&this.refreshMenuInstanceDebounced(e)},this)},refreshMenuLocation:function(a){var c=!1;b.each(g.navMenuInstanceArgs,function(b,d){a===b.theme_location&&(this.refreshMenuInstanceDebounced(d),c=!0)},this),c||d.preview.send("refresh")},refreshMenuInstance:function(e){var f,h,i,j,k,l,m,n;if(!g.navMenuInstanceArgs[e])throw new Error("unknown_instance_number");return m=g.navMenuInstanceArgs[e],n="partial-refreshable-nav-menu-"+String(e),j=a("."+n),b.isNumber(m.menu)?h=m.menu:m.theme_location&&d.has("nav_menu_locations["+m.theme_location+"]")&&(h=d("nav_menu_locations["+m.theme_location+"]").get()),h&&m.can_partial_refresh&&0!==j.length?(h=parseInt(h,10),f={nonce:g.previewCustomizeNonce,wp_customize:"on"},g.theme.active||(f.theme=g.theme.stylesheet),f[g.renderQueryVar]="1",i={},d.each(function(a,b){var c=a.get(),d=!1;d=d||/^nav_menu_locations\[/.test(b),d=d||b==="nav_menu["+String(h)+"]",d=d||/^nav_menu_item\[/.test(b)&&(!1===c||h===c.nav_menu_term_id),d&&(i[b]=c)}),f.customized=JSON.stringify(i),f[g.renderNoncePostKey]=g.renderNonceValue,l=a.extend({},m),f.wp_nav_menu_args_hash=l.args_hash,delete l.args_hash,f.wp_nav_menu_args=JSON.stringify(l),j.addClass("customize-partial-refreshing"),k=c.ajax.send(null,{data:f,url:g.requestUri}),void k.done(function(b){if(!1===b)return void d.preview.send("refresh");var c,f=j;j=a(b),j.addClass(n),j.addClass("partial-refreshable-nav-menu customize-partial-refreshing"),f.replaceWith(j),c={instanceNumber:e,wpNavArgs:l,oldContainer:f,newContainer:j},j.removeClass("customize-partial-refreshing"),a(document).trigger("customize-preview-menu-refreshed",[c])})):void d.preview.send("refresh")},refreshMenuInstanceDebounced:function(a){e[a]&&clearTimeout(e[a]),e[a]=setTimeout(b.bind(function(){this.refreshMenuInstance(a)},this),f)}},d.bind("preview-ready",function(){d.preview.bind("active",function(){d.MenusCustomizerPreview.init()})})}}(jQuery,_,wp);
\ No newline at end of file
+!function(a,b,c){"use strict";if(c&&c.customize){var d=c.customize,e={},f=200,g={},h={renderQueryVar:null,renderNonceValue:null,renderNoncePostKey:null,previewCustomizeNonce:null,requestUri:"/",theme:{active:!1,stylesheet:""},navMenuInstanceArgs:{}};d.MenusCustomizerPreview={init:function(){var a=this,c={};g=b.extend({},h),"undefined"!=typeof _wpCustomizePreviewNavMenusExports&&b.extend(g,_wpCustomizePreviewNavMenusExports),d.each(function(b,d){b.id=d,c[b.id]=!0,a.bindListener(b)}),d.preview.bind("setting",function(b){var e,f,g;b=b.slice(),e=b.shift(),f=b.shift(),g=d(e),g||(g=d.create(e,f)),g.id||(g.id=e),c[g.id]||(c[g.id]=!0,a.bindListener(g)&&g.callbacks.fireWith(g,[g(),null]))})},bindListener:function(a){var c,d;return(c=a.id.match(/^nav_menu\[(-?\d+)]$/))?(a.navMenuId=parseInt(c[1],10),a.bind(this.onChangeNavMenuSetting),!0):(c=a.id.match(/^nav_menu_item\[(-?\d+)]$/))?(a.navMenuItemId=parseInt(c[1],10),a.bind(this.onChangeNavMenuItemSetting),!0):(c=a.id.match(/^nav_menu_locations\[(.+?)]/),c?(d=c[1],a.bind(b.bind(function(){this.refreshMenuLocation(d)},this)),!0):!1)},onChangeNavMenuSetting:function(){var a=this;if(!a.navMenuId)throw new Error("Expected navMenuId property to be set.");d.MenusCustomizerPreview.refreshMenu(a.navMenuId)},onChangeNavMenuItemSetting:function(a,b){!b||!b.nav_menu_term_id||a&&b.nav_menu_term_id===a.nav_menu_term_id||d.MenusCustomizerPreview.refreshMenu(b.nav_menu_term_id),a&&a.nav_menu_term_id&&d.MenusCustomizerPreview.refreshMenu(a.nav_menu_term_id)},refreshMenu:function(a){var c=[];d.each(function(b,d){var e=d.match(/^nav_menu_locations\[(.+?)]/);e&&a===b()&&c.push(e[1])}),b.each(g.navMenuInstanceArgs,function(d,e){(a===d.menu||-1!==b.indexOf(c,d.theme_location))&&this.refreshMenuInstanceDebounced(e)},this)},refreshMenuLocation:function(a){var c=!1;b.each(g.navMenuInstanceArgs,function(b,d){a===b.theme_location&&(this.refreshMenuInstanceDebounced(d),c=!0)},this),c||d.preview.send("refresh")},refreshMenuInstance:function(e){var f,h,i,j,k,l,m,n;if(!g.navMenuInstanceArgs[e])throw new Error("unknown_instance_number");return m=g.navMenuInstanceArgs[e],n="partial-refreshable-nav-menu-"+String(e),j=a("."+n),b.isNumber(m.menu)?h=m.menu:m.theme_location&&d.has("nav_menu_locations["+m.theme_location+"]")&&(h=d("nav_menu_locations["+m.theme_location+"]").get()),h&&m.can_partial_refresh&&0!==j.length?(h=parseInt(h,10),f={nonce:g.previewCustomizeNonce,wp_customize:"on"},g.theme.active||(f.theme=g.theme.stylesheet),f[g.renderQueryVar]="1",i={},d.each(function(a,b){var c=a.get(),d=!1;d=d||/^nav_menu_locations\[/.test(b),d=d||b==="nav_menu["+String(h)+"]",d=d||/^nav_menu_item\[/.test(b)&&(!1===c||h===c.nav_menu_term_id),d&&(i[b]=c)}),f.customized=JSON.stringify(i),f[g.renderNoncePostKey]=g.renderNonceValue,l=a.extend({},m),f.wp_nav_menu_args_hash=l.args_hash,delete l.args_hash,f.wp_nav_menu_args=JSON.stringify(l),j.addClass("customize-partial-refreshing"),k=c.ajax.send(null,{data:f,url:g.requestUri}),void k.done(function(b){if(!1===b)return void d.preview.send("refresh");var c,f=j;j=a(b),j.addClass(n),j.addClass("partial-refreshable-nav-menu customize-partial-refreshing"),f.replaceWith(j),c={instanceNumber:e,wpNavArgs:l,wpNavMenuArgs:l,oldContainer:f,newContainer:j},j.removeClass("customize-partial-refreshing"),a(document).trigger("customize-preview-menu-refreshed",[c])})):void d.preview.send("refresh")},refreshMenuInstanceDebounced:function(a){e[a]&&clearTimeout(e[a]),e[a]=setTimeout(b.bind(function(){this.refreshMenuInstance(a)},this),f)}},d.bind("preview-ready",function(){d.preview.bind("active",function(){d.MenusCustomizerPreview.init()})})}}(jQuery,_,wp);
\ No newline at end of file
diff --git a/wp-includes/version.php b/wp-includes/version.php
index ea753b6d39..7724f36b53 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.3-beta4-33490';
+$wp_version = '4.3-beta4-33491';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.