Why "Smart Separator" doesn't work?

[Tao Wang <dancefire gmail com>]

Hi,

I'm using Gtk::Builder and Gtk::UIManager to create an interface.

According to the document: http://library.gnome.org/devel/gtkmm/unstable/classGtk_1_1UIManager.html

"For separators in toolbars, you can set expand="true" to turn them 
from a small, visible separator to an expanding, invisible one. 
Toolitems following an expanding separator are effectively 
right-aligned."

That is, if ui string is:

    <ui>
      <toolbar name="popup_toolbar">
        <toolitem action="">        <toolitem action="">
        <separator expend="true"/>
        <toolitem action="">      </toolbar>
    </ui>

The <separator> will be expended and push the button "action_lock" right aligned.

However, I didn't get the effect. The separator is still "a small, visible separator". The entire glade file is here: http://pastebin.com/T0njXfJ3

Could anyone tell me why the smart separator doesn't work? or is it a bug?
-- 
Regards

Tao Wang