Add `z-index` to menu-content elements

- Add `z-index` to menu-content elements to make sure they are rendered
  on top of other sidebar elements
- Bump `z-index` on menu arrow so that it is above `menu-content`

src/styles/sidebar/components/menu.scss

@@ -44,9 +44,9 @@
   // overlaps the content's border. The effect is that the menu's border is a
   // rounded rect with a notch at the top.
   position: absolute;
-  top: calc(100% - 2px);  // nb. Adjust this if changing the <svg> size.
+  top: calc(100% - 2px); // nb. Adjust this if changing the <svg> size.
   right: 0;
-  z-index: 1;
+  z-index: 2;
 }
 
 // Content area of the menu.
@@ -57,6 +57,7 @@
   font-size: $body2-font-size;
   position: absolute;
   top: calc(100% + 5px);
+  z-index: 1;
 
   &--align-left {
     left: 0;