From ac4ff301805b9abb0b09069c9fc22143db7e4dc9 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Wed, 17 May 2023 17:37:32 +0200
Subject: [PATCH 1/3] Added failing tests for null recycling key.

---
 .../VirtualizingStackPanelTests.cs            | 58 +++++++++++++++++--
 1 file changed, 54 insertions(+), 4 deletions(-)

diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index aa03a77d70..cb98552704 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -646,7 +646,7 @@ namespace Avalonia.Controls.UnitTests
         {
             // Issue #11272
             using var app = App();
-            var (_, _, itemsControl) = CreateUnrootedTarget();
+            var (_, _, itemsControl) = CreateUnrootedTarget<ItemsControl>();
             var container = new Decorator { Margin = new Thickness(100) };
             var root = new TestRoot(true, container);
             
@@ -657,6 +657,44 @@ namespace Avalonia.Controls.UnitTests
             root.LayoutManager.ExecuteLayoutPass();
         }
 
+        [Fact]
+        public void Supports_Null_Recycle_Key_When_Scrolling()
+        {
+            using var app = App();
+            var (_, scroll, itemsControl) = CreateUnrootedTarget<NonRecyclingItemsControl>();
+            var root = CreateRoot(itemsControl);
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            var firstItem = itemsControl.ContainerFromIndex(0)!;
+            scroll.Offset = new(0, 20);
+
+            Layout(itemsControl);
+
+            Assert.Null(firstItem.Parent);
+            Assert.Null(firstItem.VisualParent);
+            Assert.DoesNotContain(firstItem, itemsControl.ItemsPanelRoot!.Children);
+        }
+
+        [Fact]
+        public void Supports_Null_Recycle_Key_When_Clearing_Items()
+        {
+            using var app = App();
+            var (_, _, itemsControl) = CreateUnrootedTarget<NonRecyclingItemsControl>();
+            var root = CreateRoot(itemsControl);
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            var firstItem = itemsControl.ContainerFromIndex(0)!;
+            itemsControl.ItemsSource = null;
+
+            Layout(itemsControl);
+
+            Assert.Null(firstItem.Parent);
+            Assert.Null(firstItem.VisualParent);
+            Assert.Empty(itemsControl.ItemsPanelRoot!.Children);
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()
@@ -704,7 +742,7 @@ namespace Avalonia.Controls.UnitTests
             Optional<IDataTemplate?> itemTemplate = default,
             IEnumerable<Style>? styles = null)
         {
-            var (target, scroll, itemsControl) = CreateUnrootedTarget(items, itemTemplate);
+            var (target, scroll, itemsControl) = CreateUnrootedTarget<ItemsControl>(items, itemTemplate);
             var root = CreateRoot(itemsControl, styles);
 
             root.LayoutManager.ExecuteInitialLayoutPass();
@@ -712,9 +750,10 @@ namespace Avalonia.Controls.UnitTests
             return (target, scroll, itemsControl);
         }
 
-        private static (VirtualizingStackPanel, ScrollViewer, ItemsControl) CreateUnrootedTarget(
+        private static (VirtualizingStackPanel, ScrollViewer, T) CreateUnrootedTarget<T>(
             IEnumerable<object>? items = null,
             Optional<IDataTemplate?> itemTemplate = default)
+                where T : ItemsControl, new()
         {
             var target = new VirtualizingStackPanel();
 
@@ -732,7 +771,7 @@ namespace Avalonia.Controls.UnitTests
                 Template = ScrollViewerTemplate(),
             };
 
-            var itemsControl = new ItemsControl
+            var itemsControl = new T
             {
                 ItemsSource = items,
                 Template = new FuncControlTemplate<ItemsControl>((_, ns) => scroll.RegisterInNameScope(ns)),
@@ -806,5 +845,16 @@ namespace Avalonia.Controls.UnitTests
 
             public event NotifyCollectionChangedEventHandler? CollectionChanged;
         }
+
+        private class NonRecyclingItemsControl : ItemsControl
+        {
+            protected override Type StyleKeyOverride => typeof(ItemsControl);
+
+            protected internal override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
+            {
+                recycleKey = null;
+                return true;
+            }
+        }
     }
 }

From 89aef5f476ed1524bdcee4608a2ac174e9105f78 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Wed, 17 May 2023 17:37:49 +0200
Subject: [PATCH 2/3] Handle non-recyclable containers.

---
 src/Avalonia.Controls/VirtualizingStackPanel.cs | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/Avalonia.Controls/VirtualizingStackPanel.cs b/src/Avalonia.Controls/VirtualizingStackPanel.cs
index 7ec1808e63..7d3e6f600f 100644
--- a/src/Avalonia.Controls/VirtualizingStackPanel.cs
+++ b/src/Avalonia.Controls/VirtualizingStackPanel.cs
@@ -662,9 +662,12 @@ namespace Avalonia.Controls
             _scrollViewer?.UnregisterAnchorCandidate(element);
 
             var recycleKey = element.GetValue(RecycleKeyProperty);
-            Debug.Assert(recycleKey is not null);
 
-            if (recycleKey == s_itemIsItsOwnContainer)
+            if (recycleKey is null)
+            {
+                RemoveInternalChild(element);
+            }
+            else if (recycleKey == s_itemIsItsOwnContainer)
             {
                 element.IsVisible = false;
             }
@@ -687,9 +690,8 @@ namespace Avalonia.Controls
             Debug.Assert(ItemContainerGenerator is not null);
 
             var recycleKey = element.GetValue(RecycleKeyProperty);
-            Debug.Assert(recycleKey is not null);
-
-            if (recycleKey == s_itemIsItsOwnContainer)
+            
+            if (recycleKey is null || recycleKey == s_itemIsItsOwnContainer)
             {
                 RemoveInternalChild(element);
             }

From 8e68fe910cab960f897ef3380cac91288d892d93 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@users.noreply.github.com>
Date: Fri, 19 May 2023 16:31:38 +0200
Subject: [PATCH 3/3] Fix error introduced in merge.

---
 .../Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index ba6fb6a7dc..c5a6ecc376 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -699,7 +699,7 @@ namespace Avalonia.Controls.UnitTests
         public void ScrollIntoView_On_Effectively_Invisible_Panel_Does_Not_Create_Ghost_Elements()
         {
             var items = new[] { "foo", "bar", "baz" };
-            var (target, _, itemsControl) = CreateUnrootedTarget(items: items);
+            var (target, _, itemsControl) = CreateUnrootedTarget<ItemsControl>(items: items);
             var container = new Decorator { Margin = new Thickness(100), Child = itemsControl };
             var root = new TestRoot(true, container);