Implement indexed sort for visual children.

src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Avalonia.Media;
 using Avalonia.Platform;
 using Avalonia.Threading;
@@ -258,14 +259,25 @@ namespace Avalonia.Rendering.SceneGraph
                         }
                         else if (visualChildren.Count > 1)
                         {
-                            var sortedChildren = new IVisual[visualChildren.Count];
+                            var count = visualChildren.Count;
-                            visualChildren.CopyTo(sortedChildren, 0);
+                            var sortedChildren = new (IVisual visual, int index)[count];
 
-                            Array.Sort(sortedChildren, ZIndexComparer.ComparisonInstance);
+                            for (var i = 0; i < count; i++)
+                            {
+                                sortedChildren[i] = (visualChildren[i], i);
+                            }
+
+                            // Regular Array.Sort is unstable, we need to provide indices as well to avoid reshuffling elements.
+                            Array.Sort(sortedChildren, (lhs, rhs) =>
+                            {
+                                var result = ZIndexComparer.Instance.Compare(lhs.visual, rhs.visual);
+
+                                return result == 0 ? lhs.index.CompareTo(rhs.index) : result;
+                            });
 
                             foreach (var child in sortedChildren)
                             {
-                                var childNode = GetOrCreateChildNode(scene, child, node);
+                                var childNode = GetOrCreateChildNode(scene, child.Item1, node);
                                 Update(context, scene, (VisualNode)childNode, clip, forceRecurse);
                             }
                         }