Add transient flag for notifications which shouldn't linger in history

osu.Game.Tests/Visual/UserInterface/TestSceneNotificationOverlay.cs

@@ -83,6 +83,40 @@ namespace osu.Game.Tests.Visual.UserInterface
             waitForCompletion();
         }
 
+        [Test]
+        public void TestNormalDoesForwardToOverlay()
+        {
+            SimpleNotification notification = null!;
+
+            AddStep(@"simple #1", () => notificationOverlay.Post(notification = new SimpleNotification
+            {
+                Text = @"This shouldn't annoy you too much",
+                Transient = false,
+            }));
+
+            AddAssert("notification in toast tray", () => notification.IsInToastTray, () => Is.True);
+            AddUntilStep("wait for dismissed", () => notification.IsInToastTray, () => Is.False);
+
+            checkDisplayedCount(1);
+        }
+
+        [Test]
+        public void TestTransientDoesNotForwardToOverlay()
+        {
+            SimpleNotification notification = null!;
+
+            AddStep(@"simple #1", () => notificationOverlay.Post(notification = new SimpleNotification
+            {
+                Text = @"This shouldn't annoy you too much",
+                Transient = true,
+            }));
+
+            AddAssert("notification in toast tray", () => notification.IsInToastTray, () => Is.True);
+            AddUntilStep("wait for dismissed", () => notification.IsInToastTray, () => Is.False);
+
+            checkDisplayedCount(0);
+        }
+
         [Test]
         public void TestForwardWithFlingRight()
         {

osu.Game/Online/FriendPresenceNotifier.cs

@@ -169,6 +169,7 @@ namespace osu.Game.Online
 
             notifications.Post(new SimpleNotification
             {
+                Transient = true,
                 Icon = FontAwesome.Solid.UserPlus,
                 Text = $"Online: {string.Join(@", ", onlineAlertQueue.Select(u => u.Username))}",
                 IconColour = colours.Green,
@@ -204,6 +205,7 @@ namespace osu.Game.Online
 
             notifications.Post(new SimpleNotification
             {
+                Transient = true,
                 Icon = FontAwesome.Solid.UserMinus,
                 Text = $"Offline: {string.Join(@", ", offlineAlertQueue.Select(u => u.Username))}",
                 IconColour = colours.Red

osu.Game/Overlays/NotificationOverlayToastTray.cs

@@ -41,7 +41,7 @@ namespace osu.Game.Overlays
         [Resolved]
         private OverlayColourProvider colourProvider { get; set; } = null!;
 
-        public Action<Notification>? ForwardNotificationToPermanentStore { get; set; }
+        public required Action<Notification> ForwardNotificationToPermanentStore { get; init; }
 
         public int UnreadCount => Notifications.Count(n => !n.WasClosed && !n.Read);
 
@@ -142,8 +142,15 @@ namespace osu.Game.Overlays
             notification.MoveToOffset(new Vector2(400, 0), NotificationOverlay.TRANSITION_LENGTH, Easing.OutQuint);
             notification.FadeOut(NotificationOverlay.TRANSITION_LENGTH, Easing.OutQuint).OnComplete(_ =>
             {
+                if (notification.Transient)
+                {
+                    notification.IsInToastTray = false;
+                    notification.Close(false);
+                    return;
+                }
+
                 RemoveInternal(notification, false);
-                ForwardNotificationToPermanentStore?.Invoke(notification);
+                ForwardNotificationToPermanentStore(notification);
 
                 notification.FadeIn(300, Easing.OutQuint);
             });

osu.Game/Overlays/Notifications/Notification.cs

@@ -34,10 +34,16 @@ namespace osu.Game.Overlays.Notifications
         public abstract LocalisableString Text { get; set; }
 
         /// <summary>
-        /// Whether this notification should forcefully display itself.
+        /// Important notifications display for longer, and announce themselves at an OS level (ie flashing the taskbar).
+        /// This defaults to <c>true</c>.
         /// </summary>
         public virtual bool IsImportant => true;
 
+        /// <summary>
+        /// Transient notifications only show as a toast, and do not linger in notification history.
+        /// </summary>
+        public bool Transient { get; init; }
+
         /// <summary>
         /// Run on user activating the notification. Return true to close.
         /// </summary>