Index: src/wp-content/themes/twentyfifteen/style.css
===================================================================
--- src/wp-content/themes/twentyfifteen/style.css	(revision 30946)
+++ src/wp-content/themes/twentyfifteen/style.css	(working copy)
@@ -691,6 +691,10 @@
 	padding-right: 48px;
 }
 
+.main-navigation .current_page_ancestor .sub-menu {
+	display: block;
+}
+
 .main-navigation .menu-item-description {
 	color: #707070;
 	color: rgba(51, 51, 51, 0.7);