public class PopupNonTopmost : Popup

    {

        public static DependencyProperty TopmostProperty = Window.TopmostProperty.AddOwner(

            typeof(PopupNonTopmost),

            new FrameworkPropertyMetadata(false, OnTopmostChanged));

        public bool Topmost

        {

            get { return (bool)GetValue(TopmostProperty); }

            set { SetValue(TopmostProperty, value); }

        }

        private static void OnTopmostChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)

        {

            (obj as PopupNonTopmost).UpdateWindow();

        }

        protected override void OnOpened(EventArgs e)

        {

            UpdateWindow();

        }

        private void UpdateWindow()

        {

            var hwnd = ((HwndSource)PresentationSource.FromVisual(this.Child)).Handle;

            RECT rect;

            if (GetWindowRect(hwnd, out rect))

            {

                SetWindowPos(hwnd, Topmost ? -1 : -2, rect.Left, rect.Top, (int)this.Width, (int)this.Height, 0);

            }

        }

        #region P/Invoke imports & definitions

        [StructLayout(LayoutKind.Sequential)]

        public struct RECT

        {

            public int Left;

            public int Top;

            public int Right;

            public int Bottom;

        }

        [DllImport("user32.dll")]

        [return: MarshalAs(UnmanagedType.Bool)]

        private static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);

        [DllImport("user32", EntryPoint = "SetWindowPos")]

        private static extern int SetWindowPos(IntPtr hWnd, int hwndInsertAfter, int x, int y, int cx, int cy, int wFlags);

        #endregion

    }