此行为应解决弹出窗口定位问题。

using Microsoft.Xaml.Behaviors;
using System;
using System.Runtime.Versioning;
using System.Windows;
using System.Windows.Controls.Primitives;
using System.Windows.Interop;
using System.Windows.Media;
using System.Windows.Media.Media3D;

namespace MCS.UI.WpfUICommon.AttachedProperties
{
[SupportedOSPlatform("windows")]
public class AutoRepositionPopupBehavior : Behavior<Popup>
{
    private const int WM_MOVING = 0x0216;

    // should be moved to a helper class
    private DependencyObject GetTopmostParent(DependencyObject element)
    {
        var current = element;
        var result = element;

        while (current != null)
        {
            result = current;
            current = (current is Visual || current is Visual3D) ?
               VisualTreeHelper.GetParent(current) :
               LogicalTreeHelper.GetParent(current);
        }
        return result;
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        //AssociatedObject.LayoutUpdated += (sender, e) => Update();
        AssociatedObject.Loaded += (sender, e) =>
        {
            if (GetTopmostParent(AssociatedObject.PlacementTarget) is Window root)
            {
                var helper = new WindowInteropHelper(root);
                var hwndSource = HwndSource.FromHwnd(helper.Handle);
                if (hwndSource != null)
                {
                    hwndSource.AddHook(HwndMessageHook);
                }
            }
        };
    }

    private IntPtr HwndMessageHook(IntPtr hWnd,
            int msg, IntPtr wParam,
            IntPtr lParam, ref bool bHandled)
    {
        if (msg == WM_MOVING)
        {
            Update();
        }
        return IntPtr.Zero;
    }

    public void Update()
    {
        // force the popup to update it's position
        var mode = AssociatedObject.Placement;
        AssociatedObject.Placement = PlacementMode.Relative;
        AssociatedObject.Placement = mode;
    }
}
}

xaml中的用法：

xmlns:b="http://schemas.microsoft.com/xaml/behaviors" 
         xmlns:attachedprops="clr-namespace:YourBehaviourNameSpace"

 <Popup .... >
    <b:Interaction.Behaviors>                      
        <attachedprops:AutoRepositionPopupBehavior/>
    </b:Interaction.Behaviors>
</Popup>