the -3 values your
> 'frame-geometry' reports for external border sizes are not reproducible
> here.

My dot ctwmrc has these options commented out.  They may explain the
minus 3 pixel value detected.

 #UseThreeDIconManagers
 #UseThreeDWMap
 #UseThreeDBorders
 #UseThreeDMenus
 #UseThreeDTitles

                                            Have you ever tried building
>    with GTK-3?

I'm not able to use the gtk-3 build as it causes the tty/emacsclient session
to lockup.  The bug is documented.

The gtk-3 build is the default option from pkgsrc.

-- 
vl