Overlapping panels

I need to implement a group template that its header overlaps/flow over the group members container.
In the attached pic you can see an expanded group (its nodes are also groups) which its header (marked with blue arrow) flows over the group contained. I tried margin and padding. No success…
Please advise.

    myDiagram.groupTemplate =
      $(go.Group, "Spot",
        { isShadowed: true },
        $(go.Panel, "Auto",
            { fill: "white" },
            new go.Binding("stroke", "color")),
          $(go.Placeholder, { padding: new go.Margin(30, 10, 10, 10) })
        $(go.Panel, "Auto",
          { alignment: go.Spot.Top },
          $(go.Shape, "RoundedRectangle",
            { fill: "white" },
            new go.Binding("stroke", "color")),
            { margin: new go.Margin(5, 10) },
            new go.Binding("text"))

produces something like:

Works fine,