From 0efe0418540bf08887663d0c3052db456eb76e03 Mon Sep 17 00:00:00 2001
From: Petar Tasev <ptasev@gmail.com>
Date: Fri, 23 Jan 2026 14:30:43 -0800
Subject: [PATCH 1/2] Bring header presenter into view, and size to auto

---
 src/Avalonia.Controls/TreeViewItem.cs             | 15 ++++++---------
 .../Controls/TreeViewItem.xaml                    |  2 +-
 .../Controls/TreeViewItem.xaml                    |  2 +-
 3 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/src/Avalonia.Controls/TreeViewItem.cs b/src/Avalonia.Controls/TreeViewItem.cs
index ec1e943da0..3c4d52fdb9 100644
--- a/src/Avalonia.Controls/TreeViewItem.cs
+++ b/src/Avalonia.Controls/TreeViewItem.cs
@@ -191,16 +191,13 @@ namespace Avalonia.Controls
                     return;
                 }
 
-                if (_header != null)
+                var header = _headerPresenter ?? _header;
+                var m = header?.TransformToVisual(this);
+                if (m.HasValue)
                 {
-                    var m = _header.TransformToVisual(this);
-
-                    if (m.HasValue)
-                    {
-                        var bounds = new Rect(_header.Bounds.Size);
-                        var rect = bounds.TransformToAABB(m.Value);
-                        e.TargetRect = rect;
-                    }
+                    var bounds = new Rect(header.Bounds.Size);
+                    var rect = bounds.TransformToAABB(m.Value);
+                    e.TargetRect = rect;
                 }
             }
         }
diff --git a/src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml b/src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml
index a1b22528c9..e242c0988d 100644
--- a/src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml
+++ b/src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml
@@ -73,7 +73,7 @@
                   MinHeight="{TemplateBinding MinHeight}"
                   TemplatedControl.IsTemplateFocusTarget="True">
             <Grid Name="PART_Header"
-                  ColumnDefinitions="Auto, *">
+                  ColumnDefinitions="Auto, Auto">
               <Grid.Margin>
                 <MultiBinding Converter="{x:Static converters:TreeViewItemIndentConverter.Instance}">
                   <Binding Path="Level" RelativeSource="{RelativeSource TemplatedParent}" />
diff --git a/src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml b/src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml
index b16e69d4c9..1513166dfa 100644
--- a/src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml
+++ b/src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml
@@ -43,7 +43,7 @@
                   Focusable="True"
                   TemplatedControl.IsTemplateFocusTarget="True">
             <Grid Name="PART_Header"
-                  ColumnDefinitions="16, *">
+                  ColumnDefinitions="16, Auto">
               <Grid.Margin>
                 <MultiBinding Converter="{x:Static converters:TreeViewItemIndentConverter.Instance}">
                   <Binding Path="Level" RelativeSource="{RelativeSource TemplatedParent}" />

From 303bd0a015994ca115f5a29f481816d8cc9f61a6 Mon Sep 17 00:00:00 2001
From: Petar Tasev <ptasev@gmail.com>
Date: Fri, 23 Jan 2026 14:56:31 -0800
Subject: [PATCH 2/2] Fix null annotation

---
 src/Avalonia.Controls/TreeViewItem.cs | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/Avalonia.Controls/TreeViewItem.cs b/src/Avalonia.Controls/TreeViewItem.cs
index 3c4d52fdb9..8ef0131b19 100644
--- a/src/Avalonia.Controls/TreeViewItem.cs
+++ b/src/Avalonia.Controls/TreeViewItem.cs
@@ -192,12 +192,16 @@ namespace Avalonia.Controls
                 }
 
                 var header = _headerPresenter ?? _header;
-                var m = header?.TransformToVisual(this);
-                if (m.HasValue)
+                if (header is not null)
                 {
-                    var bounds = new Rect(header.Bounds.Size);
-                    var rect = bounds.TransformToAABB(m.Value);
-                    e.TargetRect = rect;
+                    var m = header.TransformToVisual(this);
+
+                    if (m.HasValue)
+                    {
+                        var bounds = new Rect(header.Bounds.Size);
+                        var rect = bounds.TransformToAABB(m.Value);
+                        e.TargetRect = rect;
+                    }
                 }
             }
         }