Make tabs full-width at small viewport

app/assets/stylesheets/pages/merge_requests.scss

@@ -433,6 +433,10 @@
     left: 0;
     z-index: 10;
     transition: right .15s;
+
+    @media (max-width: $screen-xs-max) {
+      right: 0;
+    }
   }
 
   &:not(.affix) .container-fluid {