For a menu mode CB with wrap_width == 0 and an active item, that item is
selected in gtk_combo_box_menu_popup. Selection causes the MenuShell to
activate and hence take a grab. This was done before the menu was popped
up. A patch distributed in Debian sid - after being proposed on our BZ -
revealed that on the 1st popup of any such ComboBox, within grab_add,
the MenuShell's toplevel's GdkWindow is NULL. This causes a Gdk-CRITICAL
assertion fail on the 1st time opening any such CB, on Debian and if
that patch were merged to GTK+. By selecting after popup, we ensure the
MenuShell is realised before its grab_add and so avoid the critical.

https://bugzilla.gnome.org/show_bug.cgi?id=771242