> Somewhere in the bowels
 > of frame.c, et.al., is an issue where text-pixels behaves differently than
 > set-frame-size which is reliable on NS where make-frame using text-pixels
 > is incorrect by the vertical scroll bar width. GTK it's off, too, but I
 > didn't analyze that further.

If you mean that with

(progn
   (setq frame-resize-pixelwise t)
   (make-frame '((width . (text-pixels . 800)))))

then

(frame-text-width)

evaluates to 784, please try the attached patch.

Thanks, martin