/// <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>