Merge pull request #6271 from MarchingCube/fix-zindex-ordering

Implement indexed sort for visual children.

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

@@ -258,14 +258,25 @@ namespace Avalonia.Rendering.SceneGraph
                         }
                         else if (visualChildren.Count > 1)
                         {
-                            var sortedChildren = new IVisual[visualChildren.Count];
-                            visualChildren.CopyTo(sortedChildren, 0);
+                            var count = visualChildren.Count;
+                            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);
                             }
                         }

tests/Avalonia.Visuals.UnitTests/Rendering/SceneGraph/SceneBuilderTests.cs

@@ -245,6 +245,34 @@ namespace Avalonia.Visuals.UnitTests.Rendering.SceneGraph
             }
         }
 
+        [Fact]
+        public void Should_Respect_Uniform_ZIndex()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                Panel panel;
+
+                var tree = new TestRoot
+                {
+                    Child = panel = new Panel()
+                };
+
+                for (var i = 0; i < 128; i++)
+                {
+                    panel.Children.Add(new Border());
+                }
+
+                var result = new Scene(tree);
+                var sceneBuilder = new SceneBuilder();
+                sceneBuilder.UpdateAll(result);
+
+                var panelNode = result.FindNode(tree.Child);
+                var expected = panel.Children.ToArray();
+                var actual = panelNode.Children.OfType<IVisualNode>().Select(x => x.Visual).ToArray();
+                Assert.Equal(expected, actual);
+            }
+        }
+
         [Fact]
         public void ClipBounds_Should_Be_In_Global_Coordinates()
         {