/// <summary>
/// Popup帮助类，解决Popup设置StayOpen="True"时，移动窗体或者改变窗体大小时，Popup不随窗体移动的问题
/// </summary>
public class PopopHelper
{
    public static DependencyObject GetPopupPlacementTarget(DependencyObject obj)
    {
        return (DependencyObject)obj.GetValue(PopupPlacementTargetProperty);
    }

    public static void SetPopupPlacementTarget(DependencyObject obj, DependencyObject value)
    {
        obj.SetValue(PopupPlacementTargetProperty, value);
    }
    
    public static readonly DependencyProperty PopupPlacementTargetProperty =
        DependencyProperty.RegisterAttached("PopupPlacementTarget", typeof(DependencyObject), typeof(PopopHelper), new PropertyMetadata(null, OnPopupPlacementTargetChanged));

    private static void OnPopupPlacementTargetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue != null)
        {
            DependencyObject popupPopupPlacementTarget = e.NewValue as DependencyObject;
            Popup pop = d as Popup;

            Window w = Window.GetWindow(popupPopupPlacementTarget);
            if (null != w)
            {
                //让Popup随着窗体的移动而移动
                w.LocationChanged += delegate
                {
                    var mi = typeof(Popup).GetMethod("UpdatePosition", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
                    mi.Invoke(pop, null);
                };
                //让Popup随着窗体的Size改变而移动位置
                w.SizeChanged += delegate
                {
                    var mi = typeof(Popup).GetMethod("UpdatePosition", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
                    mi.Invoke(pop, null);
                };
            }
        }
    }
}

<Popup x:Name="PART_Popup" AllowsTransparency="True" IsOpen="True" Placement="Bottom"
       PlacementTarget="{Binding ElementName=PART_ToggleButton}"
       HorizontalOffset="-5"
       ZUI:PopopHelper.PopupPlacementTarget="{Binding ElementName=PART_ToggleButton}"
       StaysOpen="True">
    <Border x:Name="ItemsPresenter" Background="Transparent">
        <ItemsPresenter />
    </Border>
</Popup>