When switching the selection on the GtkFileChooserButton using the combobox, a wrong a current directory is reported by gtk_file_chooser_get_current_folder_file(). Also, no current-folder-changed signal is emitted.

Here a quick example in JS, just play a bit opening the dialog, closing it etc... :

-------------------------------------------------------------------

const Lang = imports.lang;
const Mainloop = imports.mainloop;

const Gtk = imports.gi.Gtk;

Gtk.init(null, null);

let mwindow = new Gtk.Window({ opacity: 0.5 });
mwindow.connect('delete-event', Lang.bind(this, function() {
    Gtk.main_quit();
}));

let chooser = new Gtk.FileChooserButton({ action: Gtk.FileChooserAction.SELECT_FOLDER, });
mwindow.add(chooser);

Mainloop.timeout_add_seconds(1, Lang.bind(this, function() {
    let f = chooser.get_current_folder_file()
    if (f)
        log(f.get_path());
    else
        log('no selected folder');
    return true;
}));

mwindow.show_all();
Gtk.main();