Added tests for hiding diffs (and especially the last visible diff)

osu.Game.Tests/Visual/TestCaseBeatmapCarousel.cs

@@ -70,6 +70,7 @@ namespace osu.Game.Tests.Visual
 
             testRemoveAll();
             testEmptyTraversal();
+            testHiding();
         }
 
         private void ensureRandomFetchSuccess() =>
@@ -295,6 +296,40 @@ namespace osu.Game.Tests.Visual
             checkNoSelection();
         }
 
+        private void testHiding()
+        {
+            var hidingSet = createTestBeatmapSet(1);
+            hidingSet.Beatmaps[1].Hidden = true;
+            AddStep("Add set with diff 2 hidden", () => carousel.UpdateBeatmapSet(hidingSet));
+            setSelected(1, 1);
+
+            checkVisibleItemCount(true, 2);
+            advanceSelection(true);
+            checkSelected(1, 3);
+
+            setHidden(3);
+            checkSelected(1, 1);
+
+            setHidden(2, false);
+            advanceSelection(true);
+            checkSelected(1, 2);
+
+            setHidden(1);
+            checkSelected(1, 2);
+
+            setHidden(2);
+            checkNoSelection();
+
+            void setHidden(int diff, bool hidden = true)
+            {
+                AddStep((hidden ? "" : "un") + $"hide diff {diff}", () =>
+                {
+                    hidingSet.Beatmaps[diff - 1].Hidden = hidden;
+                    carousel.UpdateBeatmapSet(hidingSet);
+                });
+            }
+        }
+
         private BeatmapSetInfo createTestBeatmapSet(int i)
         {
             return new BeatmapSetInfo