bug#31968: 26.1.50; Allow to hide title bar on maximize (gtk/gnome/csd)

[martin rudalics]

> I had the opportunity to test Robert's C patch on a recent copy of the
> master branch today in a Fedora 28 VM using Gnome3, Xfce4, and KDE5.
> Setting the hide argument to `t' in the new Lisp function
> `x-set-hide-titlebar-when-maximized' worked as expected in Gnome and
> Xfce--the titlebar disappeared when I maximized the Emacs frame.

Thank you very much for testing.  Can you check whether it works when
maximizing an undecorated frame?  That is, make a new frame, in it do

(set-frame-parameter nil 'undecorated t)

then maximize it and demaximize it.  Is it still undecorated?

> The
> KDE titlebar remained in place except when putting Emacs in fullscreen
> mode.

Somehow KDE is a notorious refusnik as far as Emacs frames are
concerned.  While you're there: Can you confirm the strange resizing
behaviors of Bug#31968?  For example, does evaluating

(modify-frame-parameters nil '((left . 0) (top . 0) (width . 130) (height . 
56)))

result in a 130x56 frame?

Thanks again, martin