Issue #3200868 by codebymikey, mably: [theming] Show/Hide TOC elements entirely using CSS classes

Closes #3200868

assets/js/tocjs.js

@@ -413,8 +413,8 @@
         refreshBodyPaddingTop(tocObj);
       }
 
-      tocObj.$element.show();
-      tocObj.$container.find('a.back-to-top').show();
+      tocObj.$element.addClass('is-visible');
+      tocObj.$container.find('a.back-to-top').addClass('is-visible');
     }
     tocObj.updating = false;
   }