Shaker/Avalonia/SukiUI.Dock/ToolPinnedControl.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:dmc="using:Dock.Model.Controls"
                    xmlns:core="using:Dock.Model.Core">
  <Design.PreviewWith>
    <ToolPinnedControl Width="30" Height="400" />
  </Design.PreviewWith>

  <ControlTheme x:Key="{x:Type ToolPinnedControl}" TargetType="ToolPinnedControl">

    <Setter Property="Orientation" Value="Vertical" />

    <Setter Property="Template">
      <ControlTemplate>
        <ItemsControl ItemsSource="{TemplateBinding Items}"
                      x:DataType="dmc:IRootDock"
                      x:CompileBindings="True">
          <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
              <StackPanel Orientation="{Binding $parent[ToolPinnedControl].Orientation}"
                          DockProperties.IsDropArea="True" />
            </ItemsPanelTemplate>
          </ItemsControl.ItemsPanel>
          <ItemsControl.DataTemplates>
            <DataTemplate DataType="core:IDockable">
              <ToolPinItemControl Orientation="{Binding $parent[ToolPinnedControl].Orientation}" />
            </DataTemplate>
          </ItemsControl.DataTemplates>
        </ItemsControl>
      </ControlTemplate>
    </Setter>
  </ControlTheme>

</ResourceDictionary>