When I set the HeaderTemplate property of an AdvancedTabItem Visibility is set to Collapsed

Initially in my DataTemplate, I had a StackPanel with a TextBlock within it and everytime I started up my application what I expected to be displyed would be set to Visibility Collapsed on the StackPanel.

With the same code working on a normal TabItem as I was expecting them to work in a fairly similar way.

After some head scratching I found a helpful topic on the forum where an example of a HeaderTemplate being set. 

It turns out it is okay if I use a Grid, or not Panels at all...

Is there a specific way why your implementation doesn't work with StackPanels? And possibly why this should be avoided with normal TabItems?

<UserControl>

    <UserControl.Resources>
        <Style TargetType="{x:Type docking:AdvancedTabItem}" x:Key="MyTabStyle">
            <Setter Property="Background" Value="Orange"/>
            <Setter Property="HeaderTemplate">
                <Setter.Value>
                    <DataTemplate >
                        <!--<StackPanel Orientation="Horizontal" Margin="5" Width="30" Height="20">-->
                        <Grid>
                            <TextBlock Text="{Binding Name}"  FontSize="22" Background="HotPink" />
                        </Grid>
                        <!--</StackPanel>-->
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </UserControl.Resources>


    <Grid>
        <docking:AdvancedTabControl MinHeight="80" 
                                    CanTabsDrag="True" 
                                    ItemContainerStyle="{StaticResource MyTabStyle}" 
                                    ItemsSource="{Binding Groups}" 
                                    SelectedItem="{Binding SelectedGroup}" />

    </Grid>
</UserControl>