From d7b6a8e97a6d838f20d418b25e6eff429383d3d0 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Mon, 27 Feb 2023 18:08:32 +0100
Subject: [PATCH 1/2] Added failing test for #10398.

---
 .../TreeViewTests.cs                          | 25 +++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
index 1a9fb33b7e..2ca3495d30 100644
--- a/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/TreeViewTests.cs
@@ -100,6 +100,31 @@ namespace Avalonia.Controls.UnitTests
             Assert.All(items, x => Assert.Same(theme, x.ItemContainerTheme));
         }
 
+        [Fact]
+        public void Finds_Correct_DataTemplate_When_Application_DataTemplate_Is_Present()
+        {
+            // #10398
+            using var app = UnitTestApplication.Start();
+            
+            Avalonia.Application.Current.DataTemplates.Add(new FuncDataTemplate<object>((x, _) => new Canvas()));
+            AvaloniaLocator.CurrentMutable.Bind<IGlobalDataTemplates>().ToConstant(Avalonia.Application.Current);
+
+            var target = new TreeView
+            {
+                Template = CreateTreeViewTemplate(),
+                Items = CreateTestTreeData(),
+            };
+
+            var root = new TestRoot(target);
+
+            CreateNodeDataTemplate(target);
+            ApplyTemplates(target);
+
+            Assert.Equal(new[] { "Root" }, ExtractItemHeader(target, 0));
+            Assert.Equal(new[] { "Child1", "Child2", "Child3" }, ExtractItemHeader(target, 1));
+            Assert.Equal(new[] { "Grandchild2a" }, ExtractItemHeader(target, 2));
+        }
+
         [Fact]
         public void Root_ItemContainerGenerator_Containers_Should_Be_Root_Containers()
         {

From 5281bbcca097bbe4a8ce7d58264d02efc4df3292 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 28 Feb 2023 16:39:41 +0100
Subject: [PATCH 2/2] Search for data template on tree attachment.

When a `HeaderedItemsControl` is used in an `ItemsControl` it needs to search for an `ITreeDataTemplate` in order to populate the `Items` property. This can't be done properly until it's attached to the logical tree.

Fixes #10398
---
 src/Avalonia.Controls/ItemsControl.cs         |  9 +---
 .../Primitives/HeaderedItemsControl.cs        | 47 +++++++++++++++++++
 2 files changed, 48 insertions(+), 8 deletions(-)

diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 9483f98881..bebf4a38f6 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -383,14 +383,7 @@ namespace Avalonia.Controls
             {
                 hic.Header = item;
                 hic.HeaderTemplate = itemTemplate;
-
-                itemTemplate ??= hic.FindDataTemplate(item) ?? this.FindDataTemplate(item);
-
-                if (itemTemplate is ITreeDataTemplate treeTemplate)
-                {
-                    if (item is not null && treeTemplate.ItemsSelector(item) is { } itemsBinding)
-                        BindingOperations.Apply(hic, ItemsProperty, itemsBinding, null);
-                }
+                hic.PrepareItemContainer();
             }
         }
 
diff --git a/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs b/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
index 71ae7a5bf6..55d2ec7506 100644
--- a/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
+++ b/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
@@ -1,6 +1,8 @@
+using System;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
+using Avalonia.Data;
 using Avalonia.LogicalTree;
 
 namespace Avalonia.Controls.Primitives
@@ -10,6 +12,9 @@ namespace Avalonia.Controls.Primitives
     /// </summary>
     public class HeaderedItemsControl : ItemsControl, IContentPresenterHost
     {
+        private IDisposable? _itemsBinding;
+        private bool _prepareItemContainerOnAttach;
+
         /// <summary>
         /// Defines the <see cref="Header"/> property.
         /// </summary>
@@ -60,6 +65,17 @@ namespace Avalonia.Controls.Primitives
         /// <inheritdoc/>
         IAvaloniaList<ILogical> IContentPresenterHost.LogicalChildren => LogicalChildren;
 
+        protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToLogicalTree(e);
+
+            if (_prepareItemContainerOnAttach)
+            {
+                PrepareItemContainer();
+                _prepareItemContainerOnAttach = false;
+            }
+        }
+
         /// <inheritdoc/>
         bool IContentPresenterHost.RegisterContentPresenter(IContentPresenter presenter)
         {
@@ -81,6 +97,37 @@ namespace Avalonia.Controls.Primitives
             return false;
         }
 
+        internal void PrepareItemContainer()
+        {
+            _itemsBinding?.Dispose();
+            _itemsBinding = null;
+
+            var item = Header;
+
+            if (item is null)
+            {
+                _prepareItemContainerOnAttach = false;
+                return;
+            }
+
+            var headerTemplate = HeaderTemplate;
+
+            if (headerTemplate is null)
+            {
+                if (((ILogical)this).IsAttachedToLogicalTree)
+                    headerTemplate = this.FindDataTemplate(item);
+                else
+                    _prepareItemContainerOnAttach = true;
+            }
+
+            if (headerTemplate is ITreeDataTemplate treeTemplate &&
+                treeTemplate.Match(item) &&
+                treeTemplate.ItemsSelector(item) is { } itemsBinding)
+            {
+                _itemsBinding = BindingOperations.Apply(this, ItemsProperty, itemsBinding, null);
+            }
+        }
+
         private void HeaderChanged(AvaloniaPropertyChangedEventArgs e)
         {
             if (e.OldValue is ILogical oldChild)