Allow the notification overlay to close when all notifications are dismissed

osu.Game/Overlays/NotificationOverlay.cs

@@ -22,7 +22,7 @@ namespace osu.Game.Overlays
         private ScrollContainer scrollContainer;
         private FlowContainer<NotificationSection> sections;
 
-        [BackgroundDependencyLoader(permitNulls: true)]
+        [BackgroundDependencyLoader]
         private void load()
         {
             Width = width;
@@ -72,6 +72,13 @@ namespace osu.Game.Overlays
 
         private int runningDepth;
 
+        private void notificationClosed()
+        {
+            // hide ourselves if all notifications have been dismissed.
+            if (sections.Select(c => c.DisplayedCount).Sum() > 0)
+                State = Visibility.Hidden;
+        }
+
         public void Post(Notification notification)
         {
             Schedule(() =>
@@ -81,6 +88,8 @@ namespace osu.Game.Overlays
                 ++runningDepth;
                 notification.Depth = notification.DisplayOnTop ? runningDepth : -runningDepth;
 
+                notification.Closed += notificationClosed;
+
                 var hasCompletionTarget = notification as IHasCompletionTarget;
                 if (hasCompletionTarget != null)
                     hasCompletionTarget.CompletionTarget = Post;

osu.Game/Overlays/Notifications/Notification.cs

@@ -19,9 +19,9 @@ namespace osu.Game.Overlays.Notifications
     public abstract class Notification : Container
     {
         /// <summary>
-        /// Use requested close.
+        /// User requested close.
         /// </summary>
-        public Action Closed;
+        public event Action Closed;
 
         /// <summary>
         /// Run on user activating the notification. Return true to close.

osu.Game/Overlays/Notifications/NotificationSection.cs

@@ -24,6 +24,8 @@ namespace osu.Game.Overlays.Notifications
 
         private FlowContainer<Notification> notifications;
 
+        public int DisplayedCount => notifications.Count;
+
         public void Add(Notification notification)
         {
             notifications.Add(notification);