With respect (and forasmuch as I love anything that promotes vim) - if some program and/or theme has managed to request <= 0 size for a widget, then it's broken GTK+, not the other way around.
That said, on a quick glance at the gtk-3-22 branch, I can't see any changes in gtkwidget.c that would've broken this, but of course extra debug info may reveal that some other widget is leading to this.
