Update: focus to focus-visible (fixes #231)

Author: guywillis

less/core/nav.less

@@ -35,7 +35,7 @@ html.is-nav-bottom #wrapper {
       .u-display-none;
     }
 
-    &:not(:focus) {
+    &:not(:focus-visible) {
       height: 0;
       padding: 0;
       margin: 0;