title: StackPanel.ActualHeight is always zero
body: <p>I create a <code>StackPanel</code> in run-time and I want to measure the <code>Height</code> of the <code>StackPanel</code> like this:</p>
  
  <pre><code>StackPanel panel = new StackPanel();
  panel.Children.Add(new Button() { Width = 75, Height = 25 });
  Title = panel.ActualHeight.ToString();
  </code></pre>
  
  <p>but <code>ActualHeight</code> is alwasy zero. How can I measure the <code>Height</code> Of the <code>StackPanel</code>?</p>