Disallow hiding beatmap difficulties if only one difficulty remains

osu.Game.Tests/Visual/SongSelectV2/TestSceneSongSelectFiltering.cs

@@ -309,6 +309,24 @@ namespace osu.Game.Tests.Visual.SongSelectV2
             checkMatchedBeatmaps(3);
         }
 
+        [Test]
+        public void TestCantHideAllBeatmaps()
+        {
+            LoadSongSelect();
+            ImportBeatmapForRuleset(0);
+
+            checkMatchedBeatmaps(3);
+
+            AddStep("hide selected", () => Beatmaps.Hide(Beatmap.Value.BeatmapInfo));
+            checkMatchedBeatmaps(2);
+
+            AddStep("hide selected", () => Beatmaps.Hide(Beatmap.Value.BeatmapInfo));
+            checkMatchedBeatmaps(1);
+
+            AddAssert("hide fails", () => Beatmaps.Hide(Beatmap.Value.BeatmapInfo), () => Is.False);
+            checkMatchedBeatmaps(1);
+        }
+
         private NoResultsPlaceholder? getPlaceholder() => SongSelect.ChildrenOfType<NoResultsPlaceholder>().FirstOrDefault();
 
         private void checkMatchedBeatmaps(int expected) => AddUntilStep($"{expected} matching shown", () => Carousel.MatchedBeatmapsCount, () => Is.EqualTo(expected));

osu.Game/Beatmaps/BeatmapManager.cs

@@ -218,24 +218,37 @@ namespace osu.Game.Beatmaps
         }
 
         /// <summary>
-        /// Delete a beatmap difficulty.
+        /// Hide a beatmap difficulty.
+        /// Will fail if all difficulties are about to be hidden.
         /// </summary>
         /// <param name="beatmapInfo">The beatmap difficulty to hide.</param>
-        public void Hide(BeatmapInfo beatmapInfo)
+        public bool Hide(BeatmapInfo beatmapInfo)
         {
-            Realm.Run(r =>
+            return Realm.Run(r =>
             {
                 using (var transaction = r.BeginWrite())
                 {
                     if (!beatmapInfo.IsManaged)
                         beatmapInfo = r.Find<BeatmapInfo>(beatmapInfo.ID)!;
 
+                    if (!CanHide(beatmapInfo))
+                        return false;
+
                     beatmapInfo.Hidden = true;
                     transaction.Commit();
+                    return true;
                 }
             });
         }
 
+        public bool CanHide(BeatmapInfo beatmapInfo) => Realm.Run(r =>
+        {
+            if (!beatmapInfo.IsManaged)
+                beatmapInfo = r.Find<BeatmapInfo>(beatmapInfo.ID)!;
+
+            return beatmapInfo.BeatmapSet!.Beatmaps.Count(b => !b.Hidden) > 1;
+        });
+
         /// <summary>
         /// Restore a beatmap difficulty.
         /// </summary>

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

@@ -112,7 +112,7 @@ namespace osu.Game.Screens.Select.Carousel
             }
 
             if (manager != null)
-                hideRequested = manager.Hide;
+                hideRequested = b => manager.Hide(b);
 
             Header.Children = new Drawable[]
             {

osu.Game/Screens/SelectV2/SoloSongSelect.cs

@@ -84,7 +84,9 @@ namespace osu.Game.Screens.SelectV2
             {
                 Icon = FontAwesome.Solid.Eraser
             };
-            yield return new OsuMenuItem(WebCommonStrings.ButtonsHide.ToSentence(), MenuItemType.Destructive, () => beatmaps.Hide(beatmap));
+
+            if (beatmaps.CanHide(beatmap))
+                yield return new OsuMenuItem(WebCommonStrings.ButtonsHide.ToSentence(), MenuItemType.Destructive, () => beatmaps.Hide(beatmap));
         }
 
         protected override void OnStart()