bug#23144: shrinking windows with gtk 3.20

[martin rudalics]

> We would prefer to not include the change in Emacs 25.1 (that will be
> released from the emacs-25 branch), and instead have some workaround
> there that will be activated by the end-users, if and when they see
> problems related to this.  Can you propose such a workaround?
> Something like a call to xg_frame_resized with some reasonable
> dimensions, for example?

In emacs-25 either use something like

#if !GTK_CHECK_VERSION (3, 20, 0))
          xg_frame_resized (f, -1, -1);
#endif

or, as Matthias proposed, in xg_frame_resized write

  if (pixelwidth == -1 && pixelheight == -1)
    {
      if (FRAME_GTK_WIDGET (f) && gtk_widget_get_mapped (FRAME_GTK_WIDGET (f)))
        {
          gdk_window_get_geometry (gtk_widget_get_window (FRAME_GTK_WIDGET (f)),
                                   0, 0, &pixelwidth, &pixelheight);
          if (pixelwidth <= 1 || pixelheight <= 1)
            return;
        }
      else
        return;
    }

martin