class GoPanelAdorner : Adorner
{
  readonly GoPanel _panel;

  public GoPanelAdorner(UIElement textArea, GoPanel panel)
    : base(textArea)
  {
    _panel = panel;
    AddVisualChild(panel);
  }

  protected override int VisualChildrenCount
  {
    get { return 1; }
  }

  protected override Visual GetVisualChild(int index)
  {
    if (index != 0)
      throw new ArgumentOutOfRangeException();
    return _panel;
  }

  protected override Size ArrangeOverride(Size finalSize)
  {
    _panel.Arrange(new Rect(new Point(0, 0), finalSize));
    return new Size(_panel.ActualWidth, _panel.ActualHeight);
  }
}

public void Attach(TextArea textArea)
{
  if (textArea == null)
    throw new ArgumentNullException("textArea");
  _textArea = textArea;
  var layer = AdornerLayer.GetAdornerLayer(textArea);
  _adorner = new GoPanelAdorner(textArea, this);
  if (layer != null)
    layer.Add(_adorner);
}

<Style TargetType="{x:Type AttachedUI:GoPanel}">
  <Setter Property="SnapsToDevicePixels" Value="True"/>
  <Setter Property="OverridesDefaultStyle" Value="True"/>
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type AttachedUI:GoPanel}">
        <Border
          HorizontalAlignment="Right"
          VerticalAlignment="Top"
          <Border.Background>
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