Hide sub-navigation on mobile to stop screen flickering while scrolling

Ref: sourcegraph#3666

The sidebar appears above the content on screen sizes below 900px wide.

As we scroll down the page, sub-menus in the sidebar become active.

This pushes content down which deactivates the sub-menu, causing a loop.

src/styles/page.scss

@@ -66,8 +66,14 @@
     > li > ul {
         padding-left: 0;
 
-        ul:not(.active) {
+        ul {
             display: none;
+
+            @media (min-width: 900px) {
+                &.active {
+                    display: block;
+                }
+            }
         }
     }