gistfile1.txt

  var hwndSource = HwndSource.FromHwnd(handle);
  if (hwndSource != null)
  {
      hwndSource.AddHook(WindowProc);
  }

WinAPI.ShowWindowAsync(new WindowInteropHelper(OverlayForm).Handle, 0);

private IntPtr WindowProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
        {
            switch (msg)
            {
                case 0x0003:
                    uint uFlags = WinAPI.SWP_NOACTIVATE | WinAPI.SWP_NOZORDER;

                    int parent = Application.ActiveWindow.Hwnd;
                    System.Drawing.Rectangle rect = new System.Drawing.Rectangle();
                    WinAPI.GetWindowRect((IntPtr)parent, ref rect);
                    WinAPI.SetWindowPos((uint)hwnd.ToInt32(), parent, 0, 0, rect.Right, rect.Bottom, uFlags);
                    break;
            }
            return (IntPtr)0;
        }

[DllImport("user32.dll")]
public static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hWnd, int id);