> (make-frame `(
 > 	      (parent-frame . ,(window-frame))
 > 	      (child-frame-border-width . 0)
 > 	      (internal-border-width . 200)))
 >
 > I would expect the border width to only fall back if
 > child-frame-border-width is nil, since 0 is a useful setting.

It's not entirely trivial to do that (frame parameters are a pain in
this regard) but please have a look at the attached patch (tested on a
GTK build only).  Alexander, please have a look too.

Thanks, martin