Bring header presenter into view, and size to auto

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;
                 }
             }
         }

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}" />

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}" />