Get relative position of Popup to its parent.

PopupHelper.cs

public static class PopupHelper
{
    [DllImport("User32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);

    [StructLayout(LayoutKind.Sequential)]
    private struct RECT
    {
        public int Left;
        public int Top;
        public int Right;
        public int Bottom;

        public static implicit operator Rect(RECT rect) => new(rect.Left, rect.Top, rect.Right - rect.Left, rect.Bottom - rect.Top);
    }

    public static Vector GetRelativePositionToParent(Popup popup)
    {
        if ((PresentationSource.FromVisual(popup.Child) is HwndSource popupSource) &&
            GetWindowRect(popupSource.Handle, out RECT popupRect))
        {
            Point parentPosition = ((Visual)popup.Parent).PointToScreen(new Point(0, 0));
            return ((Rect)popupRect).Location - parentPosition;
        }
        return default;
    }
}

See https://stackoverflow.com/questions/9183222/determine-popup-placement-relative-to-target