Set double click on presenter instead of whole header

src/Avalonia.Controls/TreeViewItem.cs

@@ -45,6 +45,7 @@ namespace Avalonia.Controls
 
         private TreeView? _treeView;
         private Control? _header;
+        private Control? _headerPresenter;
         private int _level;
         private bool _templateApplied;
         private bool _deferredBringIntoViewFlag;
@@ -255,15 +256,16 @@ namespace Avalonia.Controls
 
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
-            if (_header is InputElement previousInputMethod)
+            if (_headerPresenter is InputElement previousInputMethod)
             {
                 previousInputMethod.DoubleTapped -= HeaderDoubleTapped;
             }
 
             _header = e.NameScope.Find<Control>("PART_Header");
+            _headerPresenter = e.NameScope.Find<Control>("PART_HeaderPresenter");
             _templateApplied = true;
 
-            if (_header is InputElement im)
+            if (_headerPresenter is InputElement im)
             {
                 im.DoubleTapped += HeaderDoubleTapped;
             }

src/Avalonia.Themes.Fluent/Controls/TreeViewItem.xaml

@@ -75,7 +75,6 @@
                   MinHeight="{TemplateBinding MinHeight}"
                   TemplatedControl.IsTemplateFocusTarget="True">
             <Grid Name="PART_Header"
-                  Background="Transparent"
                   ColumnDefinitions="Auto, *"
                   Margin="{TemplateBinding Level, Mode=OneWay, Converter={StaticResource TreeViewItemLeftMarginConverter}}">
               <Panel Name="PART_ExpandCollapseChevronContainer"
@@ -88,6 +87,7 @@
               <ContentPresenter Name="PART_HeaderPresenter"
                                 Grid.Column="1"
                                 Focusable="False"
+                                Background="Transparent"
                                 Content="{TemplateBinding Header}"
                                 ContentTemplate="{TemplateBinding HeaderTemplate}"
                                 HorizontalAlignment="{TemplateBinding HorizontalAlignment}"

src/Avalonia.Themes.Simple/Controls/TreeViewItem.xaml

@@ -44,18 +44,19 @@
                   Focusable="True"
                   TemplatedControl.IsTemplateFocusTarget="True">
             <Grid Name="PART_Header"
-                  Background="Transparent"
                   Margin="{TemplateBinding Level,
                                            Mode=OneWay,
                                            Converter={StaticResource LeftMarginConverter}}"
                   ColumnDefinitions="16, *">
               <ToggleButton Name="PART_ExpandCollapseChevron"
                             Focusable="False"
+                            Background="Transparent"
                             IsChecked="{TemplateBinding IsExpanded,
                                                         Mode=TwoWay}"
                             Theme="{StaticResource SimpleTreeViewItemToggleButtonTheme}" />
               <ContentPresenter Name="PART_HeaderPresenter"
                                 Grid.Column="1"
+                                Background="Transparent"
                                 Padding="{TemplateBinding Padding}"
                                 HorizontalContentAlignment="{TemplateBinding HorizontalAlignment}"
                                 Content="{TemplateBinding Header}"