How to show every LayoutAnchorable objects that are children of a LayoutAnchorablePane by default

Question

I'm creating LayoutAnchorable dynamically in code and making them a child of a LayoutAnchorablePane. The one I add as a last child is being shown but the other ones is hidden and looks like they are tabbed. How can I show them both by default like they are stacked ?

LayoutAnchorable layoutAnchorable = new LayoutAnchorable
{
    Content = listBox,
    Title = child1,
    ContentId = _selectedSchema.Key,
    IsActive = true,
    IsSelected = true,
    CanFloat = true,
    CanAutoHide = false,
    CanClose = true,
    CanHide = false,
};

//layoutAnchorable.DockWidth = new GridLength(200, GridUnitType.Pixel);

if (!panelXml.Children.Select(x => x.ContentId).Contains(layoutAnchorable.ContentId))
{
    panelXml.Children.Add(layoutAnchorable);
}