TabItem headers: Content doesn't stretch if the actual tab header does

Question

I have a TabControl and am attempting to get the headers to look nice. I have the following XAML:

(In Resources.xaml:)

<DataTemplate x:Key="ClosableTabItemTemplate">
    <DockPanel LastChildFill="True" MinWidth="200" Height="20" HorizontalAlignment="Stretch" behaviors:MouseClickBehavior.ClickCommand="{Binding Path=CloseCommand}">
        <Image Source="{Binding Path=Image}" Height="16" VerticalAlignment="Center" />
        <Button Background="Transparent"
                  BorderBrush="Transparent"
                  Command="{Binding Path=CloseCommand}"
                  Content="X"
                  Cursor="Hand"
                  DockPanel.Dock="Right"
                  Focusable="False" 
                  FontSize="9"
                  FontWeight="Bold"
                  Margin="3,0,0,0"
                  Padding="0"
                  VerticalAlignment="Center"
                  VerticalContentAlignment="Center"
                  Width="16" Height="16" />
        <TextBlock Text="{Binding Path=Header}" DockPanel.Dock="Left" VerticalAlignment="Center" />
    </DockPanel>
</DataTemplate>

(In MainWindow.xaml:)

<TabControl Grid.Column="1"
            ItemsSource="{Binding Path=Tabs}"
            ItemTemplate="{DynamicResource ClosableTabItemTemplate}" />

Here is a visual example of my problem:

I like the actual width of the tabs for now, but the fact that my dockpanel for the header isn't filling up all the space is bothersome. Same thing happens if I use a Grid, too (presumably any container control).

Answer 1

ContentPresenter.HorizontalAlignment in the default style for TabItem is bound to ItemsControl.HorizontalContentAlignment. Here it is, taken from Aero.NormalColor.xaml:

<ContentPresenter Name="Content"
                  ContentSource="Header"
                  SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                  HorizontalAlignment="{Binding Path=HorizontalContentAlignment,RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"
                  VerticalAlignment="{Binding Path=VerticalContentAlignment,RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"
                  RecognizesAccessKey="True"/>

Setting TabControl.HorizontalContentAlignment to Stretch fixes the issue for me.

<TabControl HorizontalContentAlignment="Stretch" />