Fix slider control point connections not being updated

osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointConnectionPiece.cs

@@ -20,7 +20,17 @@ namespace osu.Game.Rulesets.Osu.Edit.Blueprints.Sliders.Components
 
         private readonly Path path;
         private readonly Slider slider;
-        private readonly int controlPointIndex;
+        private int controlPointIndex;
+
+        public int ControlPointIndex
+        {
+            get => controlPointIndex;
+            set
+            {
+                controlPointIndex = value;
+                updateConnectingPath();
+            }
+        }
 
         private IBindable<Vector2> sliderPosition;
         private IBindable<int> pathVersion;

osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointVisualiser.cs

@@ -66,6 +66,17 @@ namespace osu.Game.Rulesets.Osu.Edit.Blueprints.Sliders.Components
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
+                    // If inserting in the the path (not appending),
+                    // update indices of existing connections after insert location
+                    if (e.NewStartingIndex < Pieces.Count)
+                    {
+                        foreach (var connection in Connections)
+                        {
+                            if (connection.ControlPointIndex >= e.NewStartingIndex)
+                                connection.ControlPointIndex += e.NewItems.Count;
+                        }
+                    }
+
                     for (int i = 0; i < e.NewItems.Count; i++)
                     {
                         var point = (PathControlPoint)e.NewItems[i];
@@ -82,12 +93,25 @@ namespace osu.Game.Rulesets.Osu.Edit.Blueprints.Sliders.Components
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
+                    int oldSize = Pieces.Count;
+
                     foreach (var point in e.OldItems.Cast<PathControlPoint>())
                     {
                         Pieces.RemoveAll(p => p.ControlPoint == point);
                         Connections.RemoveAll(c => c.ControlPoint == point);
                     }
 
+                    // If removing before the end of the path,
+                    // update indices of connections after remove location
+                    if (e.OldStartingIndex + e.OldItems.Count < oldSize)
+                    {
+                        foreach (var connection in Connections)
+                        {
+                            if (connection.ControlPointIndex >= e.OldStartingIndex)
+                                connection.ControlPointIndex -= e.OldItems.Count;
+                        }
+                    }
+
                     break;
             }
         }