[toc2] ensure sidebar doesn't cover menus

Author: jcb91

src/jupyter_contrib_nbextensions/nbextensions/toc2/main.css

@@ -74,7 +74,7 @@ padding-left: 20px;
 }
 
 #toc-wrapper {
-  z-index: 100;
+  z-index: 90;
   position: fixed !important;
   display: flex;
   flex-direction: column;