2017-10-18 20:34 GMT+02:00 martin rudalics <rudalics@gmx.at>:
> I am not sure: I have run only w32 and gtk3.

Then you customized something you never saw in action.

Or the default value of border-width is different for w32 and for gtk3:

;;; w32
(assoc 'border-width (frame-parameters))
(border-width . 2)

;;; gtk3
(assoc 'border-width (frame-parameters))
(border-width . 0)

This is with 'emacs -Q' in both cases.
I didn't change it myself.

Fabrice