> It needs to work under X11 as well before I'd consider integrating the
> feature. BTW, another detail is: does this property mean "stay above
> every other GUI window", or "stay above the current Emacs frame", or
> "stay above all other frames owned by this Emacs process"?
>
> The latter seems more useful. The former is probably what your
> code does. In X11, there's a feature which could do something like
> the 2nd option, which is to mark a frame as being "transient", in which
> case most window managers will try to keep it "above its parent window",
> at least if that transient window is sufficiently small.
>
>
> Stefan

I did try to implement the 2nd option ("stay above the current Emacs
frame") too by using the existing frame parameter "parent-id".
Something like:

In proc w32_createwindow:
      if (f->output_data.w32->parent_desc != FRAME_X_DISPLAY_INFO (f)->root_window)
        {
          long style;

          SetWindowLong (hwnd, GWL_HWNDPARENT, (long)f->output_data.w32->parent_desc);
          style = GetWindowLong (hwnd, GWL_STYLE);
          style &= ~(WS_MINIMIZEBOX);
          SetWindowLong (hwnd, GWL_STYLE, style);
        }
Comment out the following line in x-create-frame:
     /* f->output_data.w32->parent_desc = FRAME_W32_DISPLAY_INFO (f)->root_window;*/
And also something need to be taken care in Fdelete_frame.

It works. But sometimes it has problem in redisplaying the parent window
(parent window is not painted sometimes). And the child frame (window)
doesn't have icon/button in the windows taskbar too.

In GTK+, there is a API to do the 2nd option:
gtk_window_set_transient_for. In Windows, this API actually is
implemented by calling SetWindowLong with GWL_HWNDPARENT too.

But the MSDN says:
   You must not call SetWindowLong with the GWL_HWNDPARENT index to
   change the parent of a child window. Instead, use the SetParent
   function.
But SetParent is not what we need.