How to change the overlapping order of TabItems in WPF TabControl

Question

I have created vertical TabItems with a Path object. The selected TabItem overlaps the unselected TabItems, this works fine. The overlapping is done by setting a negative margin in the TabItem Template.

For the unselected TabItems right now a TabItem is overlapped by the TabItem below. For example in the picture Tab 4 overlaps Tab 3 and Tab 3 overlaps Tab 2.

I would like to change the overlapping order for the unselected Tab Items, so that an unselected TabItem overlaps the TabItem below and is overlapped by the TabItem above, eg Tab 2 overlaps Tab 3 and Tab 3 overlaps Tab 4.

I have tried to set the FlowDirection property of TabPanel, but this doesn't work.

How can I achieve this? Any help is appreciated. Thanks in advance!

Wrong overlapping of unselected TabItems:

XAML-Code:

<Style x:Key="styleMainNavTabControl" TargetType="{x:Type TabControl}">
    <Setter Property="TabStripPlacement" Value="Left" />
    <Setter Property="SnapsToDevicePixels" Value="true"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type TabControl}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="200"/>
                    </Grid.ColumnDefinitions>
                    <Border Grid.Column="0" Background="White" BorderBrush="Black" BorderThickness="0,0,1,0" Padding="20">
                        <ContentPresenter ContentSource="SelectedContent" />
                    </Border>
                    <Border Grid.Column="1" Padding="0,30,10,0" Background="#F7F3F7">
                        <TabPanel Panel.ZIndex="1" Margin="-1,0,0,0" FlowDirection="RightToLeft" IsItemsHost="True" Background="Transparent"/>
                    </Border>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<Style x:Key="styleMainNavTabItem" TargetType="{x:Type TabItem}">
    <Setter Property="MinHeight" Value="90" />
    <Setter Property="FontSize" Value="14" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type TabItem}">
                <Grid Margin="0,0,0,-35">
                    <Path Name="TabPath" Stroke="Black" StrokeThickness="1" Fill="LightGray" Data="M 0,0 a 10,10 0 0 0 10,10 h 150 a 20,20 0 0 1 20,20 v 60 a 20,20 0 0 1 -20,20 h -150 a 10,10 0 0 0 -10,10 z" />
                    <ContentPresenter ContentSource="Header" Margin="10,2,10,2" VerticalAlignment="Center" TextElement.Foreground="#FF000000"/>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsSelected" Value="True">
                        <Setter Property="Panel.ZIndex" Value="100" />
                        <Setter TargetName="TabPath" Property="Fill" Value="White" />
                        <Setter TargetName="TabPath" Property="Data" Value="M 0,0 a 10,10 0 0 0 10,10 h 150 a 20,20 0 0 1 20,20 v 60 a 20,20 0 0 1 -20,20 h -150 a 10,10 0 0 0 -10,10" />
                    </Trigger>
                    <Trigger Property="IsSelected" Value="False">
                        <Setter Property="Panel.ZIndex" Value="90" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>

Answer 1

I think the easiest way to do this is with a MultiBinding/Converter that uses the Parent's ItemsContainerGenerator to determine the ZIndex . It would look something like so:

public class TabZIndexConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var tabItem = values[0] as TabItem;
        var tabControl = values[1] as TabControl;
        if (tabItem == null || tabControl == null) return Binding.DoNothing;

        var count = (int)values[2];

        var index = tabControl.ItemContainerGenerator.IndexFromContainer(tabItem);

        return count - index;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

You would then change your TabItem style to set the ZIndex in a setter (and remove the second trigger, as it will disable this setter):

<Style x:Key="styleMainNavTabItem" TargetType="{x:Type TabItem}">
    <Setter Property="Panel.ZIndex">
        <Setter.Value>
            <MultiBinding Converter="{StaticResource tabZIndexConverter}">
                <Binding RelativeSource="{RelativeSource Self}" />
                <Binding RelativeSource="{RelativeSource AncestorType={x:Type TabControl}}" />
                <Binding Path="Items.Count" RelativeSource="{RelativeSource AncestorType={x:Type TabControl}}" />
            </MultiBinding>
        </Setter.Value>
    </Setter>
    ...
</Style>