Fix beatmap carousel not preloading panels when off-screen

osu.Game/Screens/Select/BeatmapCarousel.cs

@@ -96,18 +96,19 @@ public partial class BeatmapCarousel : CompositeDrawable, IKeyBindingHandler<Glo
         /// <summary>
         /// Extend the range to retain already loaded pooled drawables.
         /// </summary>
-        private const float distance_offscreen_before_unload = 1024;
+        private const float distance_offscreen_before_unload = 2048;
 
         /// <summary>
         /// Extend the range to update positions / retrieve pooled drawables outside of visible range.
         /// </summary>
-        private const float distance_offscreen_to_preload = 512; // todo: adjust this appropriately once we can make set panel contents load while off-screen.
+        private const float distance_offscreen_to_preload = 768;
 
         /// <summary>
         /// Whether carousel items have completed asynchronously loaded.
         /// </summary>
         public bool BeatmapSetsLoaded { get; private set; }
 
+        [Cached]
         protected readonly CarouselScrollContainer Scroll;
 
         private readonly NoResultsPlaceholder noResultsPlaceholder;
@@ -1251,7 +1252,7 @@ protected override void PerformSelection()
             }
         }
 
-        protected partial class CarouselScrollContainer : UserTrackingScrollContainer<DrawableCarouselItem>
+        public partial class CarouselScrollContainer : UserTrackingScrollContainer<DrawableCarouselItem>
         {
             private bool rightMouseScrollBlocked;
 

osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmapSet.cs

@@ -5,11 +5,14 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using osu.Framework.Allocation;
+using osu.Framework.Extensions.IEnumerableExtensions;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.Cursor;
+using osu.Framework.Graphics.Primitives;
 using osu.Framework.Graphics.UserInterface;
 using osu.Framework.Utils;
 using osu.Game.Beatmaps;
@@ -46,6 +49,8 @@ public partial class DrawableCarouselBeatmapSet : DrawableCarouselItem, IHasCont
 
         private MenuItem[]? mainMenuItems;
 
+        private double timeSinceUnpool;
+
         [Resolved]
         private BeatmapManager manager { get; set; } = null!;
 
@@ -54,6 +59,7 @@ protected override void FreeAfterUse()
             base.FreeAfterUse();
 
             Item = null;
+            timeSinceUnpool = 0;
 
             ClearTransforms();
         }
@@ -92,13 +98,21 @@ protected override void Update()
                 // algorithm for this is taken from ScrollContainer.
                 // while it doesn't necessarily need to match 1:1, as we are emulating scroll in some cases this feels most correct.
                 Y = (float)Interpolation.Lerp(targetY, Y, Math.Exp(-0.01 * Time.Elapsed));
+
+            loadContentIfRequired();
         }
 
+        private CancellationTokenSource? loadCancellation;
+
         protected override void UpdateItem()
         {
+            loadCancellation?.Cancel();
+            loadCancellation = null;
+
             base.UpdateItem();
 
             Content.Clear();
+            Header.Clear();
 
             beatmapContainer = null;
             beatmapsLoadTask = null;
@@ -107,32 +121,8 @@ protected override void UpdateItem()
                 return;
 
             beatmapSet = ((CarouselBeatmapSet)Item).BeatmapSet;
-
-            DelayedLoadWrapper background;
-            DelayedLoadWrapper mainFlow;
-
-            Header.Children = new Drawable[]
-            {
-                // Choice of background image matches BSS implementation (always uses the lowest `beatmap_id` from the set).
-                background = new DelayedLoadWrapper(() => new SetPanelBackground(manager.GetWorkingBeatmap(beatmapSet.Beatmaps.MinBy(b => b.OnlineID)))
-                {
-                    RelativeSizeAxes = Axes.Both,
-                }, 200)
-                {
-                    RelativeSizeAxes = Axes.Both
-                },
-                mainFlow = new DelayedLoadWrapper(() => new SetPanelContent((CarouselBeatmapSet)Item), 50)
-                {
-                    RelativeSizeAxes = Axes.Both
-                },
-            };
-
-            background.DelayedLoadComplete += fadeContentIn;
-            mainFlow.DelayedLoadComplete += fadeContentIn;
         }
 
-        private void fadeContentIn(Drawable d) => d.FadeInFromZero(150);
-
         protected override void Deselected()
         {
             base.Deselected();
@@ -190,6 +180,56 @@ private void updateBeatmapDifficulties()
             }
         }
 
+        [Resolved]
+        private BeatmapCarousel.CarouselScrollContainer scrollContainer { get; set; } = null!;
+
+        private void loadContentIfRequired()
+        {
+            Quad containingSsdq = scrollContainer.ScreenSpaceDrawQuad;
+
+            // Using DelayedLoadWrappers would only allow us to load content when on screen, but we want to preload while off-screen
+            // to provide a better user experience.
+
+            // This is tracking time that this drawable is updating since the last pool.
+            // This is intended to provide a debounce so very fast scrolls (from one end to the other of the carousel)
+            // don't cause huge overheads.
+            //
+            // We increase the delay based on distance from centre, so the beatmaps the user is currently looking at load first.
+            float timeUpdatingBeforeLoad = 50 + Math.Abs(containingSsdq.Centre.Y - ScreenSpaceDrawQuad.Centre.Y) / containingSsdq.Height * 100;
+
+            Debug.Assert(Item != null);
+
+            // A load is already in progress if the cancellation token is non-null.
+            if (loadCancellation != null)
+                return;
+
+            timeSinceUnpool += Time.Elapsed;
+
+            // We only trigger a load after this set has been in an updating state for a set amount of time.
+            if (timeSinceUnpool <= timeUpdatingBeforeLoad)
+                return;
+
+            loadCancellation = new CancellationTokenSource();
+
+            LoadComponentsAsync(new CompositeDrawable[]
+            {
+                // Choice of background image matches BSS implementation (always uses the lowest `beatmap_id` from the set).
+                new SetPanelBackground(manager.GetWorkingBeatmap(beatmapSet.Beatmaps.MinBy(b => b.OnlineID)))
+                {
+                    RelativeSizeAxes = Axes.Both,
+                },
+                new SetPanelContent((CarouselBeatmapSet)Item)
+                {
+                    Depth = float.MinValue,
+                    RelativeSizeAxes = Axes.Both,
+                }
+            }, drawables =>
+            {
+                Header.AddRange(drawables);
+                drawables.ForEach(d => d.FadeInFromZero(150));
+            }, loadCancellation.Token);
+        }
+
         private void updateBeatmapYPositions()
         {
             if (beatmapContainer == null)