Comment 13 for bug 1242801

 mcgyver
For oxygen, edit the normally already existing file "/usr/share/themes/oxygen-gtk/gtk-2.0/gtkrc" and change
    GtkComboBox::appears-as-list = 1
into
    GtkComboBox::appears-as-list = 0

(I think you should be able to also achieve that by editing some $HOME/.gtkrc, but which file exactly is to be edited is distro-specific)

I will not add a GUI option for that, since it is a gtk bug in the first place.