WPF Expander - Horizontal Alignment set to Right, but jumps to left on when expanded

Question

I've got a WPF Expander that is right aligned. That bit shows correctly on the screen, but when you expand it the expander and the text "show/hide historic data" jumps to the left of the screen. Close the expander and it returns to the correct (right) alignment.

I've tried various approaches but am unable to find why this is happening or more importantly how to keep an expanded expander right aligned

  <Grid Grid.Row="1"                    
    HorizontalAlignment="Stretch">
     <Grid.RowDefinitions>
      <RowDefinition Height="auto"/>
      <RowDefinition Height="auto"/>                               
     </Grid.RowDefinitions>
     <Grid.ColumnDefinitions>
       <ColumnDefinition Width="*" />
     </Grid.ColumnDefinitions>

      <StackPanel Grid.Row="0"  >
            <TextBlock Style="{StaticResource SubHeader}" Text="Historic Data Sets" />                                               
           <Rectangle Style="{StaticResource HeaderLine}"/>
      </StackPanel>

       <Expander Header="show/hide historic data"
                 Grid.Column="1"
                 IsEnabled="True" 
                 VerticalAlignment="Center" 
                 HorizontalAlignment="Right" 
                 ExpandDirection="Down">       

   <!-- some other stuff in the second row such as grid... -->
</Expander>

Answer 1

YOu can take control of the Header by explictly marking it as a TextBlock and then change itś HorizontalAlignment property:

<Expander.Header>
    <TextBlock 
        Width="200"
        HorizontalAlignment="Right"
        Text="Show information.."></TextBlock>
</Expander.Header>