title: WPF ToolBar Separator shrinks to nothing when inside a StackPanel
body: <p>Given the very simple wpf app</p>
  
  <pre><code>&lt;Window x:Class="Window1"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      Title="Window1" Height="300" Width="800"&gt;
      &lt;Grid&gt;
          &lt;ToolBar Height="50" &gt;
              &lt;MenuItem Header="Test1" /&gt;
              &lt;MenuItem Header="Test2" /&gt;
  
              &lt;StackPanel Orientation="Horizontal"&gt;
                  &lt;Separator /&gt;
                  &lt;MenuItem Header="Test3" /&gt;
                  &lt;MenuItem Header="Test4" /&gt;
                  &lt;MenuItem Header="Test5" /&gt;
              &lt;/StackPanel&gt;
          &lt;/ToolBar&gt;
      &lt;/Grid&gt;
  &lt;/Window&gt;
  </code></pre>
  
  <p>The Separator element shrinks to nothing. If I put the Separator just before the StackPanel begins, it will show up. Why does this happen? Is there a style setting that can be applied somewhere to avoid this?</p>