How do I get the index of a Widget in onPreviewDrop() in a VerticalPanel?

[confile]

I use the example here: https://gwt-dnd.appspot.com/#InsertPanelExample with only one column. 

The column is a VerticalPanel which contains many Widgets.

Using the VerticalPanelDropController I can register: 

  @Override

  public void onDrop(DragContext context) {

    super.onDrop(context);

    final Widget widget = context.draggable;

    final int newIndex = columnPanel.getWidgetIndex(widget);

  }

@Override

public void onPreviewDrop(DragContext context) throws VetoDragException {

super.onPreviewDrop(context);

int newIndex = ???

}

in onDrop() I can get the new position of a widget in the columnPanel with columnPanel.getWidgetIndex(widget) but in onPreviewDrop() I cannot do it because the widget is not inside the columnPanel at this time.

How do I get the target index where a dropped Widget should be dropped into the columnPanel in onPreviewDrop()? I want to know where the widget should be inserted before it is inserted. This way I can cancel the insertion.

[confile]

@Fred Sauer Do you have any idea how to solve this?