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

<Style xmlns="https://github.com/avaloniaui"
       Selector="TreeView">
  <Setter Property="Background" Value="Transparent" />
  <Setter Property="BorderBrush" Value="Transparent" />
  <Setter Property="BorderThickness" Value="0" />
  <Setter Property="Padding" Value="0" />
  <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
  <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
  <Setter Property="ScrollViewer.IsScrollChainingEnabled" Value="True" />
  <Setter Property="Template">
    <ControlTemplate>
      <Border BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="{TemplateBinding BorderThickness}"
              CornerRadius="{TemplateBinding CornerRadius}">
        <ScrollViewer Background="{TemplateBinding Background}"
                      HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
                      VerticalScrollBarVisibility="{TemplateBinding (ScrollViewer.VerticalScrollBarVisibility)}"
                      IsScrollChainingEnabled="{TemplateBinding (ScrollViewer.IsScrollChainingEnabled)}"
                      AllowAutoHide="{TemplateBinding (ScrollViewer.AllowAutoHide)}">
          <ItemsPresenter Name="PART_ItemsPresenter"
                          Items="{TemplateBinding Items}"
                          ItemsPanel="{TemplateBinding ItemsPanel}"
                          Margin="{TemplateBinding Padding}" />
        </ScrollViewer>
      </Border>
    </ControlTemplate>
  </Setter>
</Style>