[fix] sidebar style (argoproj#15652)

Signed-off-by: ymktmk <[email protected]>
Co-authored-by: Blake Pettersson <[email protected]>

Author: ymktmk

ui/src/app/sidebar/sidebar.scss

@@ -15,7 +15,7 @@ $deselected-text: #818d94;
     color: white;
     background-color: #0f2733;
     overflow: auto;
-    z-index: 0;
+    z-index: 2;
 
     &__container {
         padding: 0 5px;