messageList: Floor preferred height of LabelExpanderLayout

This fixes the flickering when expanding/collapsing an expandable
message inside a NotificationMessageGroup.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3012>

js/ui/messageList.js

@@ -277,7 +277,7 @@ const LabelExpanderLayout = GObject.registerClass({
             ];
         }
 
-        return [min, nat];
+        return [Math.floor(min), Math.floor(nat)];
     }
 });