Set z-index of dropdown to 9999. (#9126)

app/javascript/styles/mastodon/components.scss

@@ -1669,6 +1669,7 @@ a.account__display-name {
   padding: 4px 0;
   border-radius: 4px;
   box-shadow: 2px 4px 15px rgba($base-shadow-color, 0.4);
+  z-index: 9999;
 
   ul {
     list-style: none;