WPF Expander.Header horizontal stretch

Question

I have an Expander in Wpf. In the header I have Label on left aligned and want to have a button on the right site. I use the following XAML:

<Expander HorizontalAlignment="Stretch" IsExpanded="True">
    <Expander.Header >
        <Grid HorizontalAlignment="Stretch" Background="Aqua" Margin="0">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>
            <Label Grid.Column="0" Content="Label on the left site"/>
            <Button Grid.Column="1" Content="Button on the right"/>
         </Grid>
    </Expander.Header>
    <Label Content="Some Content"/>
</Expander>

But that does not work. The button in the header is aligned to the left, next to the label. Can anyone explain me how to do it right?

Answer 1

Expander header content presenter has horizontal alignment set to Left.

You can change it to Stretch using OneWayToSource binding of HorizontalAlignment (which is by default Stretch for Grid) like this:

<Expander>
    <Expander.Header>
        <Grid Background="Yellow">
            <TextBlock Text="Header"
                       HorizontalAlignment="{Binding HorizontalAlignment, RelativeSource={RelativeSource AncestorType=ContentPresenter}, Mode=OneWayToSource}" />
        </Grid>
    </Expander.Header>
</Expander>

P.S.: it took me more than it should to understand the solution of accepted answer, so I decide to save the time for future readers.