After fixing bug 147358, the default behavior in the GTK+ combox is such that, when opening, the selected option is not in the position of the combo any more, unlike the general behavior in GTK+. In GTK+ this is an intentional behavior agreed after a long discussion: https://bugzilla.gnome.org/show_bug.cgi?id=72695 Briefing. https://bugzilla.gnome.org/show_bug.cgi?id=129463#c0 I think we should revert the fix in bug 147358.
I think the empty space is by far more annoying than not having the currently selected item under the mouse when the selected item is one of the first ones in the menu.
So, in summary, not behaving like GTK+ in this regard is not a bug for me, but a feature. But if other GTK+ port maintainers disagree with me, I won't be opposed to roll the fix out, and bring back the broken behaviour.