How to set Expander Direction to slide horizontally instead of vertically?
<p>I know that the <code>System.Windows.Controls.Expander</code> is another "headered" content control in WPF just similar to a <code>System.Windows.Controls.GroupBox</code>. One advantage is that <code>Expanders</code> have the ability to hide its content by collapsing, and showing content by expanding.</p>

<p>My question is, what if I want my <code>Expander</code> to slide horizontally from left to right or right to left instead of vertically? and let's say I have the below <code>Expander</code>:</p>

<pre><code>&lt;StackPanel x:Name="RightPanel"&gt;
    &lt;Expander x:Name="ExportExpander"&gt;
        &lt;StackPanel&gt;
            &lt;TextBlock Name="x" Text="Element One"/&gt;
            &lt;Button Name="y" Content="Element Two"/&gt;
        &lt;/StackPanel&gt;
    &lt;/Expander&gt;
&lt;/StackPanel&gt; 
</code></pre>