Index: src/wp-content/themes/twentyfifteen/js/functions.js
===================================================================
--- src/wp-content/themes/twentyfifteen/js/functions.js	(revision 30956)
+++ src/wp-content/themes/twentyfifteen/js/functions.js	(working copy)
@@ -13,6 +13,10 @@
 	// Add dropdown toggle that display child menu items.
 	$( '.main-navigation .page_item_has_children > a, .main-navigation .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' );
+
 	$( '.dropdown-toggle' ).click( function( e ) {
 		var _this = $( this );
 		e.preventDefault();
Index: src/wp-content/themes/twentyfifteen/style.css
===================================================================
--- src/wp-content/themes/twentyfifteen/style.css	(revision 30956)
+++ src/wp-content/themes/twentyfifteen/style.css	(working copy)
@@ -682,6 +682,7 @@
 
 .main-navigation .current_page_item > a,
 .main-navigation .current-menu-item > a,
+.main-navigation .current-menu-ancestor > a,
 .main-navigation .current_page_ancestor > a {
 	font-weight: 700;
 }