Consider "combo offsets" as legacy logic and separate from combo information

osu.Game.Rulesets.Catch/Beatmaps/CatchBeatmapConverter.cs

@@ -38,7 +38,6 @@ namespace osu.Game.Rulesets.Catch.Beatmaps
                         RepeatCount = curveData.RepeatCount,
                         X = positionData?.X ?? 0,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
                         LegacyLastTickOffset = (obj as IHasLegacyLastTickOffset)?.LegacyLastTickOffset ?? 0
                     }.Yield();
 
@@ -49,7 +48,6 @@ namespace osu.Game.Rulesets.Catch.Beatmaps
                         Samples = obj.Samples,
                         Duration = endTime.Duration,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
                     }.Yield();
 
                 default:
@@ -58,7 +56,6 @@ namespace osu.Game.Rulesets.Catch.Beatmaps
                         StartTime = obj.StartTime,
                         Samples = obj.Samples,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
                         X = positionData?.X ?? 0
                     }.Yield();
             }

osu.Game.Rulesets.Catch/Objects/CatchHitObject.cs

@@ -67,8 +67,6 @@ namespace osu.Game.Rulesets.Catch.Objects
 
         public virtual bool NewCombo { get; set; }
 
-        public int ComboOffset { get; set; }
-
         public Bindable<int> IndexInCurrentComboBindable { get; } = new Bindable<int>();
 
         public int IndexInCurrentCombo

osu.Game.Rulesets.Osu/Beatmaps/OsuBeatmapConverter.cs

@@ -40,7 +40,7 @@ namespace osu.Game.Rulesets.Osu.Beatmaps
                         RepeatCount = curveData.RepeatCount,
                         Position = positionData?.Position ?? Vector2.Zero,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
+                        LegacyBeatmapComboOffset = (original as IHasLegacyBeatmapComboOffset)?.LegacyBeatmapComboOffset ?? 0,
                         LegacyLastTickOffset = (original as IHasLegacyLastTickOffset)?.LegacyLastTickOffset,
                         // prior to v8, speed multipliers don't adjust for how many ticks are generated over the same distance.
                         // this results in more (or less) ticks being generated in <v8 maps for the same time duration.
@@ -55,7 +55,7 @@ namespace osu.Game.Rulesets.Osu.Beatmaps
                         EndTime = endTimeData.EndTime,
                         Position = positionData?.Position ?? OsuPlayfield.BASE_SIZE / 2,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
+                        LegacyBeatmapComboOffset = (original as IHasLegacyBeatmapComboOffset)?.LegacyBeatmapComboOffset ?? 0,
                     }.Yield();
 
                 default:
@@ -65,7 +65,7 @@ namespace osu.Game.Rulesets.Osu.Beatmaps
                         Samples = original.Samples,
                         Position = positionData?.Position ?? Vector2.Zero,
                         NewCombo = comboData?.NewCombo ?? false,
-                        ComboOffset = comboData?.ComboOffset ?? 0,
+                        LegacyBeatmapComboOffset = (original as IHasLegacyBeatmapComboOffset)?.LegacyBeatmapComboOffset ?? 0,
                     }.Yield();
             }
         }

osu.Game.Rulesets.Osu/Objects/OsuHitObject.cs

@@ -14,7 +14,7 @@ using osu.Game.Rulesets.Scoring;
 
 namespace osu.Game.Rulesets.Osu.Objects
 {
-    public abstract class OsuHitObject : HitObject, IHasComboInformation, IHasPosition
+    public abstract class OsuHitObject : HitObject, IHasComboInformation, IHasPosition, IHasLegacyBeatmapComboOffset
     {
         /// <summary>
         /// The radius of hit objects (ie. the radius of a <see cref="HitCircle"/>).
@@ -73,14 +73,6 @@ namespace osu.Game.Rulesets.Osu.Objects
 
         public virtual bool NewCombo { get; set; }
 
-        public readonly Bindable<int> ComboOffsetBindable = new Bindable<int>();
-
-        public int ComboOffset
-        {
-            get => ComboOffsetBindable.Value;
-            set => ComboOffsetBindable.Value = value;
-        }
-
         public Bindable<int> IndexInCurrentComboBindable { get; } = new Bindable<int>();
 
         public virtual int IndexInCurrentCombo
@@ -105,6 +97,10 @@ namespace osu.Game.Rulesets.Osu.Objects
             set => LastInComboBindable.Value = value;
         }
 
+        public int LegacyBeatmapComboOffset { get; set; }
+
+        public int LegacyBeatmapComboIndex { get; set; }
+
         protected OsuHitObject()
         {
             StackHeightBindable.BindValueChanged(height =>

osu.Game.Tests/Gameplay/TestSceneHitObjectAccentColour.cs

@@ -82,7 +82,6 @@ namespace osu.Game.Tests.Gameplay
         private class TestHitObjectWithCombo : ConvertHitObject, IHasComboInformation
         {
             public bool NewCombo { get; set; }
-            public int ComboOffset => 0;
 
             public Bindable<int> IndexInCurrentComboBindable { get; } = new Bindable<int>();
 

osu.Game/Beatmaps/BeatmapProcessor.cs

@@ -37,7 +37,7 @@ namespace osu.Game.Beatmaps
                 if (obj.NewCombo)
                 {
                     obj.IndexInCurrentCombo = 0;
-                    obj.ComboIndex = (lastObj?.ComboIndex ?? 0) + obj.ComboOffset + 1;
+                    obj.ComboIndex = (lastObj?.ComboIndex ?? 0) + 1;
 
                     if (lastObj != null)
                         lastObj.LastInCombo = true;
@@ -48,6 +48,16 @@ namespace osu.Game.Beatmaps
                     obj.ComboIndex = lastObj.ComboIndex;
                 }
 
+                if (obj is IHasLegacyBeatmapComboOffset legacyObj)
+                {
+                    var lastLegacyObj = (IHasLegacyBeatmapComboOffset)lastObj;
+
+                    if (obj.NewCombo)
+                        legacyObj.LegacyBeatmapComboIndex = (lastLegacyObj?.LegacyBeatmapComboIndex ?? 0) + legacyObj.LegacyBeatmapComboOffset + 1;
+                    else if (lastLegacyObj != null)
+                        legacyObj.LegacyBeatmapComboIndex = lastLegacyObj.LegacyBeatmapComboIndex;
+                }
+
                 lastObj = obj;
             }
         }

osu.Game/Beatmaps/Formats/LegacyBeatmapEncoder.cs

@@ -292,10 +292,11 @@ namespace osu.Game.Beatmaps.Formats
 
             if (hitObject is IHasCombo combo)
             {
-                type = (LegacyHitObjectType)(combo.ComboOffset << 4);
-
                 if (combo.NewCombo)
                     type |= LegacyHitObjectType.NewCombo;
+
+                if (hitObject is IHasLegacyBeatmapComboOffset comboOffset)
+                    type |= (LegacyHitObjectType)(comboOffset.LegacyBeatmapComboOffset << 4);
             }
 
             switch (hitObject)

osu.Game/Rulesets/Objects/Legacy/Catch/ConvertHit.cs

@@ -13,7 +13,5 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
         public float X { get; set; }
 
         public bool NewCombo { get; set; }
-
-        public int ComboOffset { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Legacy/Catch/ConvertHitObjectParser.cs

@@ -18,21 +18,16 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
         }
 
         private bool forceNewCombo;
-        private int extraComboOffset;
 
         protected override HitObject CreateHit(Vector2 position, bool newCombo, int comboOffset)
         {
             newCombo |= forceNewCombo;
-            comboOffset += extraComboOffset;
-
             forceNewCombo = false;
-            extraComboOffset = 0;
 
             return new ConvertHit
             {
                 X = position.X,
                 NewCombo = newCombo,
-                ComboOffset = comboOffset
             };
         }
 
@@ -40,16 +35,12 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
                                                   List<IList<HitSampleInfo>> nodeSamples)
         {
             newCombo |= forceNewCombo;
-            comboOffset += extraComboOffset;
-
             forceNewCombo = false;
-            extraComboOffset = 0;
 
             return new ConvertSlider
             {
                 X = position.X,
                 NewCombo = FirstObject || newCombo,
-                ComboOffset = comboOffset,
                 Path = new SliderPath(controlPoints, length),
                 NodeSamples = nodeSamples,
                 RepeatCount = repeatCount
@@ -58,11 +49,6 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
 
         protected override HitObject CreateSpinner(Vector2 position, bool newCombo, int comboOffset, double duration)
         {
-            // Convert spinners don't create the new combo themselves, but force the next non-spinner hitobject to create a new combo
-            // Their combo offset is still added to that next hitobject's combo index
-            forceNewCombo |= FormatVersion <= 8 || newCombo;
-            extraComboOffset += comboOffset;
-
             return new ConvertSpinner
             {
                 Duration = duration

osu.Game/Rulesets/Objects/Legacy/Catch/ConvertSlider.cs

@@ -13,7 +13,5 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
         public float X { get; set; }
 
         public bool NewCombo { get; set; }
-
-        public int ComboOffset { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Legacy/Catch/ConvertSpinner.cs

@@ -17,7 +17,5 @@ namespace osu.Game.Rulesets.Objects.Legacy.Catch
         public float X => 256; // Required for CatchBeatmapConverter
 
         public bool NewCombo { get; set; }
-
-        public int ComboOffset { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertHit.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// Legacy osu! Hit-type, used for parsing Beatmaps.
     /// </summary>
-    internal sealed class ConvertHit : ConvertHitObject, IHasPosition, IHasCombo
+    internal sealed class ConvertHit : ConvertHitObject, IHasPosition, IHasCombo, IHasLegacyBeatmapComboOffset
     {
         public Vector2 Position { get; set; }
 
@@ -19,6 +19,8 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
 
         public bool NewCombo { get; set; }
 
-        public int ComboOffset { get; set; }
+        public int LegacyBeatmapComboOffset { get; set; }
+
+        int IHasLegacyBeatmapComboOffset.LegacyBeatmapComboIndex { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertHitObjectParser.cs

@@ -32,7 +32,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
             {
                 Position = position,
                 NewCombo = FirstObject || newCombo,
-                ComboOffset = comboOffset
+                LegacyBeatmapComboOffset = comboOffset
             };
         }
 
@@ -49,7 +49,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
             {
                 Position = position,
                 NewCombo = FirstObject || newCombo,
-                ComboOffset = comboOffset,
+                LegacyBeatmapComboOffset = comboOffset,
                 Path = new SliderPath(controlPoints, length),
                 NodeSamples = nodeSamples,
                 RepeatCount = repeatCount

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertSlider.cs

@@ -9,7 +9,7 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
     /// <summary>
     /// Legacy osu! Slider-type, used for parsing Beatmaps.
     /// </summary>
-    internal sealed class ConvertSlider : Legacy.ConvertSlider, IHasPosition, IHasCombo
+    internal sealed class ConvertSlider : Legacy.ConvertSlider, IHasPosition, IHasCombo, IHasLegacyBeatmapComboOffset
     {
         public Vector2 Position { get; set; }
 
@@ -19,6 +19,8 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
 
         public bool NewCombo { get; set; }
 
-        public int ComboOffset { get; set; }
+        public int LegacyBeatmapComboOffset { get; set; }
+
+        int IHasLegacyBeatmapComboOffset.LegacyBeatmapComboIndex { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Legacy/Osu/ConvertSpinner.cs

@@ -22,7 +22,5 @@ namespace osu.Game.Rulesets.Objects.Legacy.Osu
         public float Y => Position.Y;
 
         public bool NewCombo { get; set; }
-
-        public int ComboOffset { get; set; }
     }
 }

osu.Game/Rulesets/Objects/Types/IHasCombo.cs

@@ -12,10 +12,5 @@ namespace osu.Game.Rulesets.Objects.Types
         /// Whether the HitObject starts a new combo.
         /// </summary>
         bool NewCombo { get; }
-
-        /// <summary>
-        /// When starting a new combo, the offset of the new combo relative to the current one.
-        /// </summary>
-        int ComboOffset { get; }
     }
 }

osu.Game/Rulesets/Objects/Types/IHasLegacyBeatmapComboOffset.cs

@@ -0,0 +1,24 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Rulesets.Objects.Types
+{
+    /// <summary>
+    /// A type of <see cref="HitObject"/> that has a combo index with arbitrary offsets applied to use when retrieving legacy beatmap combo colours.
+    /// This is done in stable for hitobjects to skip combo colours from the beatmap skin (known as "colour hax").
+    /// See https://osu.ppy.sh/wiki/en/osu%21_File_Formats/Osu_%28file_format%29#type for more information.
+    /// </summary>
+    public interface IHasLegacyBeatmapComboOffset
+    {
+        /// <summary>
+        /// The legacy offset of the new combo relative to the current one, when starting a new combo.
+        /// </summary>
+        int LegacyBeatmapComboOffset { get; }
+
+        /// <summary>
+        /// The combo index with the <see cref="LegacyBeatmapComboOffset"/> applied,
+        /// to use for legacy beatmap skins to decide on the combo colour.
+        /// </summary>
+        int LegacyBeatmapComboIndex { get; set; }
+    }
+}