Right width of wpf expander header
<p>I've found an <a href="https://stackoverflow.com/questions/680498/how-can-i-make-a-wpf-expander-stretch">solution</a> of my problem. </p>

<pre><code>Width=”{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=x:Type Expander}}, Path=ActualWidth}”
</code></pre>

<p>But I've got not correct width. (the width of the toggle button is not taken into consideration)
Is there any solutions in XAML for this?</p>

<p><a href="https://i.stack.imgur.com/YLCsn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YLCsn.png" alt="Result"></a></p>

<pre><code>&lt;Page x:Class="Restore.Page1"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:Restore"
      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"
      Title="Page1"&gt;

    &lt;Grid Background="AliceBlue" HorizontalAlignment="Stretch"&gt;


        &lt;Expander Name="EXPANDER_NAME"&gt;
            &lt;Expander.Header&gt;
                &lt;Grid Background="Red" Width="{Binding ElementName=EXPANDER_NAME, Path=ActualWidth}"&gt;
                    &lt;Grid.ColumnDefinitions&gt;
                        &lt;ColumnDefinition Width="Auto"/&gt;
                        &lt;ColumnDefinition Width="30"/&gt;
                    &lt;/Grid.ColumnDefinitions&gt;
                    &lt;TextBlock Text="sdfsfsd" Grid.Column="0" /&gt;
                    &lt;Button Content="OK" Grid.Column="1" /&gt;
                &lt;/Grid&gt;
            &lt;/Expander.Header&gt;
        &lt;/Expander&gt;

    &lt;/Grid&gt;
&lt;/Page&gt;
</code></pre>