I have an application that creates a combo box in a dialog and populates the selection list with a liststore.  The first click on the selection arrow opens up the selection list which is too big for the screen so the list has an up scroll on the top of the list and a down scroll on the bottom of the list and scrolling works fine.
Subsequent clicks on the selection arrow do not show the scroll buttons and do not allow scrolling and hence there is no way to get to some of the selection list.  The only way to get around the problem is to cancel the dialog and open it up again and then the combo box selection list will work again on the 1st click.

A related problem also occurs some of the time which I have not been able to find a pattern for.  Occasionally, when I open the dialog with the combobox and then click on the selection arrow, the list will flash up and go away.  If I then click on the selection arrow again the previous problem occurs with no scrolling.

This application was converted from a pygtk application that never had either problem.

Here is how I am creating the combo box:'

        self.connection_name = Gtk.ComboBox.new_with_model_and_entry(conn_name_liststore)
        self.connection_name.set_entry_text_column(0)
        self.connection_name.get_child().connect('changed', connection_name_callback)
        self.connection_name.get_child().set_completion(completion)
        self.connection_name.show()

Line wrap is wrapping the lines above but in the code the statements are each on their own line.