Place your
WindowsFormHost
inside the Border control:
<StackPanel Panel.ZIndex="2" Name="pnlLeftMenu" Orientation="Horizontal" Height="400" HorizontalAlignment="Left" Margin="-150,0,0,0">
<Border BorderBrush="#AF1719" BorderThickness="1" Width="150" Background="#4C808080" >
</Border>
<Grid>
<Button Name="btnLeftMenuHide" Width="48" Height="48" Click="btnLeftMenuHide_Click" Content="Hide" Visibility="Hidden"/>
<Button Name="btnLeftMenuShow" Width="48" Height="48" Click="btnLeftMenuShow_Click" Content="Show"/>
</Grid>
</StackPanel>