public class CustomRibbonView : RadRibbonView
{
    private ContentPresenter popupContent;

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        this.popupContent = this.GetTemplateChild("SelectedTabContentPopup") as ContentPresenter;
    }

    protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
    {
        base.OnRenderSizeChanged(sizeInfo);
        if (popupContent != null)
        {
            double popupContentPadding = 16;
            popupContent.MaxWidth = this.ActualWidth - popupContentPadding;
        }
    }
}