title: Issue with child of custom Decorator class in WPF
body: <p>I need a custom border that renders a little differently than a normal border. I made a class that inherited from Decorator as follows</p>
  
  <pre><code>class BetterBorder : Decorator
  {
      protected override Size ArrangeOverride(Size arrangeSize)
      {
          return arrangeSize;
      }
  
      protected override void OnRender(DrawingContext dc)
      {
          //these values are calculated elsewhere
          dc.DrawGeometry(backgroundBrush, borderPen, pathGeometry);
      }
  }
  //Properties and helper methods below this
  </code></pre>
  
  <p>All of this works fine until I try to add a child to the control, the control can be added but is not visible and seems to be moved off BetterBorders visible client area. If I inherit from Border everything works fine, what am I missing?</p>