Fix star rating no longer updating from mod setting changes after re-entering song select

osu.Game.Tests/Beatmaps/TestSceneBeatmapDifficultyCache.cs

@@ -114,6 +114,30 @@ public void TestStarDifficultyChangesOnModSettings()
             AddUntilStep($"star difficulty -> {BASE_STARS + 1.75}", () => starDifficultyBindable.Value.Stars == BASE_STARS + 1.75);
         }
 
+        [Test]
+        public void TestStarDifficultyChangesOnModSettingsCorrectlyTrackAcrossReferenceChanges()
+        {
+            OsuModDoubleTime dt = null;
+
+            AddStep("set computation function", () => difficultyCache.ComputeDifficulty = lookup =>
+            {
+                var modRateAdjust = (ModRateAdjust)lookup.OrderedMods.SingleOrDefault(mod => mod is ModRateAdjust);
+                return new StarDifficulty(BASE_STARS + modRateAdjust?.SpeedChange.Value ?? 0, 0);
+            });
+
+            AddStep("change selected mod to DT", () => SelectedMods.Value = new[] { dt = new OsuModDoubleTime { SpeedChange = { Value = 1.5 } } });
+            AddUntilStep($"star difficulty -> {BASE_STARS + 1.5}", () => starDifficultyBindable.Value.Stars == BASE_STARS + 1.5);
+
+            AddStep("change DT speed to 1.25", () => dt.SpeedChange.Value = 1.25);
+            AddUntilStep($"star difficulty -> {BASE_STARS + 1.25}", () => starDifficultyBindable.Value.Stars == BASE_STARS + 1.25);
+
+            AddStep("reconstruct DT mod with same settings", () => SelectedMods.Value = new[] { dt = (OsuModDoubleTime)dt.DeepClone() });
+            AddUntilStep($"star difficulty -> {BASE_STARS + 1.25}", () => starDifficultyBindable.Value.Stars == BASE_STARS + 1.25);
+
+            AddStep("change DT speed to 1.25", () => dt.SpeedChange.Value = 2);
+            AddUntilStep($"star difficulty -> {BASE_STARS + 2}", () => starDifficultyBindable.Value.Stars == BASE_STARS + 2);
+        }
+
         [Test]
         public void TestStarDifficultyAdjustHashCodeConflict()
         {

osu.Game/Beatmaps/BeatmapDifficultyCache.cs

@@ -76,7 +76,10 @@ protected override void LoadComplete()
             currentMods.BindValueChanged(mods =>
             {
                 // A change in bindable here doesn't guarantee that mods have actually changed.
-                if (mods.OldValue.SequenceEqual(mods.NewValue))
+                // However, we *do* want to make sure that the mod *references* are the same;
+                // `SequenceEqual()` without a comparer would fall back to `IEquatable`.
+                // Failing to ensure reference equality can cause setting change tracking to fail later.
+                if (mods.OldValue.SequenceEqual(mods.NewValue, ReferenceEqualityComparer.Instance))
                     return;
 
                 modSettingChangeTracker?.Dispose();