I was wondering if it is possible to give xfwm titlebar negative vertical offset. It seems like it isn't right now but maybe in the future?
That is (kind of) like what Gnome/GTK3 is going for with client-side decorations. It will be interesting to see how the Xfce devs deal with this.
I don't believe this is currently possible with Xfce mostly because the Window decorations (xfwm = title, buttons) are separate from the widget decorations (gtk = menu bar).
Yes, I kind of like what Gnome team is doing with a titlebar. Maybe this could be some kind of alternative, save some space but keep a classic menu bar. We'll see how a transition will go.