Avalonia/samples/XamlTestApplicationPcl/TreeView.paml

<Style xmlns="https://github.com/perspex" Selector="TreeView">
  <Setter Property="BorderBrush" Value="Black"/>
  <Setter Property="BorderThickness" Value="1"/>
  <Setter Property="Template">
    <ControlTemplate>
      <Border Background="{TemplateBinding Background}"
              BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="{TemplateBinding BorderThickness}"
              Padding="4">
        <ScrollViewer CanScrollHorizontally="True">
          <ItemsPresenter Name="PART_ItemsPresenter"
                          Items="{TemplateBinding Items}"
                          ItemsPanel="{TemplateBinding ItemsPanel}"
                          MemberSelector="{TemplateBinding MemberSelector}"/>
        </ScrollViewer>
      </Border>
    </ControlTemplate>
  </Setter>
</Style>