Place expander button on the far right of header

I know I am missing something but how can I place the expander button on the far right of group panel?

here is the group template:

public groupExpandedTemplate(): go.Group {
  return $(go.Group,
      layout: $(go.TreeLayout, {
        angle: 0,
        comparer: go.LayoutVertex.smartComparer,
        alignment: go.TreeLayout.AlignmentCenterSubtrees,
        nodeSpacing: 5,
        nodeIndent: 0,
        nodeIndentPastParent: 0,
        layerSpacing: 10,
        layerSpacingParentOverlap: 0,
        sorting: go.TreeLayout.SortingForwards,
        rowSpacing: 10,
        rowIndent: 0,
        setsPortSpot: true,
        setsChildPortSpot: true,
        treeStyle: go.TreeLayout.StyleLayered
      selectionAdorned: false,
      selectable: false,
      selectionChanged: (thisPart: go.Part) => {
        this.interactionService.onSelectionChanged(thisPart as go.Node);
    $(go.Shape, 'Rectangle',
        fill: 'transparent',
        stroke: 'green'
          stretch: go.GraphObject.Horizontal,
          background: 'lightgray'
          alignment: go.Spot.TopLeft,
          name: 'OPERATOR_LABEL',
          overflow: go.TextBlock.OverflowEllipsis,
          margin: 4,
          maxLines: 1,
          editable: true,
          isMultiline: false,
          stroke: getComputedStyle(document.body).getPropertyValue('--text-primary'),
          textValidation: (tb: go.TextBlock, oldstr: string, newstr: string) => {
            return this.interactionService.validateStepName(oldstr, newstr);
        new go.Binding('text', 'label').makeTwoWay(),
        new go.Binding('font', 'active', (active) => (active ? 'bold 14px Lato-Regular' : '14px Lato-Regular'))
            alignment: go.Spot.TopRight,
            margin: 4
      $(go.Placeholder, { padding: 10 }),

You have 'Horizontal' panel with the SubGraphExpanderButton.

Use a Spot panel instead, and align the pieces based on where you want them:

Actually, if you want to draw what your ideal end state is, I might be able to do it for you.