notifications: fix hide/show

app/assets/javascripts/app/controllers/widget/online_notification.coffee

@@ -178,12 +178,12 @@ class App.OnlineNotificationWidget extends App.Controller
     return if !@access()
     $(window).on 'keydown.notifications', @listNavigate
     @shown = true
-    @el.show()
+    @el.addClass 'is-visible'
 
   hide: =>
     $(window).off 'keydown.notifications'
     @shown = false
-    @el.hide()
+    @el.removeClass 'is-visible'
 
   stopPropagation: (e) ->
     e.stopPropagation()

app/assets/stylesheets/zammad.scss

@@ -3715,10 +3715,13 @@ footer {
     max-width: 400px;
     min-width: 350px;
     position: absolute;
-    display: flex;
     flex-direction: column;
     @extend .zIndex-5;
     
+    &.is-visible {
+      display: flex;
+    }
+    
     &.is-empty .popover-notificationsHeader {
       box-shadow: none;
     }