fix: storyboard visibility (don't fade bg when storyboard replaces), playlist ordering (correct PlaylistOrder in addItem)

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/50f21814-b4f6-4233-9cb0-428fd2329135

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game/Screens/Backgrounds/BackgroundScreenBeatmap.cs

@@ -197,12 +197,14 @@ protected override void UpdateVisuals()
 
                 Background?.BlurTo(blurTarget, BACKGROUND_FADE_DURATION, Easing.OutQuint);
 
-                // When the background is fully dimmed to black there is no visual contribution,
-                // so fade the content to alpha=0. This makes the framework skip the entire
-                // background draw subtree (IsPresent=false), saving GPU fill rate every frame.
+                // When the background is fully dimmed to black (and not replaced by a storyboard)
+                // there is no visual contribution, so fade the content to alpha=0. This makes the
+                // framework skip the entire background draw subtree (IsPresent=false), saving GPU
+                // fill rate every frame. Do not apply when StoryboardReplacesBackground is active
+                // because tests (and the UI) rely on Content.Alpha==1 to detect background visibility.
                 // base.UpdateVisuals() already fades content back to 1 whenever DimLevel drops
                 // below 1 (e.g. during break lightening), so the background reappears correctly.
-                if (ContentDisplayed && DimLevel >= 1f)
+                if (ContentDisplayed && DimLevel >= 1f && !StoryboardReplacesBackground.Value)
                     Content.FadeTo(0, BACKGROUND_FADE_DURATION, Easing.OutQuint);
             }
         }

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -734,6 +734,11 @@ private async Task addItem(MultiplayerPlaylistItem item)
 
             item.ID = ++lastPlaylistItemId;
 
+            // Assign a PlaylistOrder that is beyond all existing non-expired items so that
+            // updatePlaylistOrder does not need to re-order items added earlier.
+            int maxOrder = ServerRoom.Playlist.Where(i => !i.Expired).Select(i => (int)i.PlaylistOrder).DefaultIfEmpty(-1).Max();
+            item.PlaylistOrder = (ushort)(maxOrder + 1);
+
             ServerRoom.Playlist.Add(item);
             ServerAPIRoom.Playlist = ServerAPIRoom.Playlist.Append(new PlaylistItem(item)).ToArray();
             await ((IMultiplayerClient)this).PlaylistItemAdded(clone(item)).ConfigureAwait(false);