DockingManager.LayoutItemTemplateSelector is applied twice because...

[Mona04]

The TemplateSelector which is bind to DockingManager.LayoutItemTemplateSelector has SelectTemplate() like below.

public override DataTemplate SelectTemplate(object item, DependencyObject container)
{
    return null;
}

When it is first fired, item is ContentPresenter and container has in Content other ContentPresenter which has an actual item in Content.

When second, it worked as expected.

I think it is because of line 763 in generic.xaml.

<Style TargetType="{x:Type avalonDockControls:LayoutDocumentControl}">
		<Setter Property="Template">
			<Setter.Value>
				<ControlTemplate TargetType="{x:Type avalonDockControls:LayoutDocumentControl}">
					<Border
						Background="{TemplateBinding Background}"
						BorderBrush="{TemplateBinding BorderBrush}"
						BorderThickness="{TemplateBinding BorderThickness}">
						<ContentPresenter
							Content="{Binding LayoutItem.View, RelativeSource={RelativeSource TemplatedParent}}"
// should deleted
							ContentTemplate="{Binding LayoutItem.View.ContentTemplate, RelativeSource={RelativeSource TemplatedParent}}" 
// should deleted 
							ContentTemplateSelector="{Binding LayoutItem.View.ContentTemplateSelector, RelativeSource={RelativeSource TemplatedParent}}" />
					</Border>
				</ControlTemplate>
			</Setter.Value>
		</Setter>
	</Style>

Or I think LayoutItem.View should not create ContentPresenter and add an override function OnApplyTemplate() of LayoutItem like below.

  public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            // Template should have ContentPresenter with a name of  PART_Content
            _view= Template.FindName("PART_Content", this) as ContentPresenter;
            LayoutElement.Root.Manager?.InternalAddLogicalChild(_view);
        }

and generic.xaml is revised like below.

   <Style TargetType="{x:Type dockctrl:LayoutDocumentControl}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type dockctrl:LayoutDocumentControl}">
                    <Border
						Background="{TemplateBinding Background}"
						BorderBrush="{TemplateBinding BorderBrush}"
						BorderThickness="{TemplateBinding BorderThickness}">
                        <!--ContentPresenter
							Content="{Binding LayoutItem.View, RelativeSource={RelativeSource TemplatedParent}}"/-->
                        <ContentPresenter x:Name="PART_Content" 
                            Content="{Binding LayoutItem.LayoutElement.Content,  RelativeSource={RelativeSource TemplatedParent}}"
							ContentTemplate="{Binding LayoutItem.LayoutElement.Root.Manager.LayoutItemTemplate, RelativeSource={RelativeSource TemplatedParent}}" 
							ContentTemplateSelector="{Binding LayoutItem.LayoutElement.Root.Manager.LayoutItemTemplateSelector, RelativeSource={RelativeSource TemplatedParent}}" />
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>