The diffrent btween toplevel window and popup window

[Eric <eric_zhang newsoft com cn>]

Dear all:

    I'am new to gtk, and I have a problem about the diffrent between toplevel window and popup window showing.

    1. when the popup window showing, it's show the window quickly( no white screen first )
    2. when the toplevel window showing, first it's showing a whilte screen ,then showing other widget, why 
,how to avoid it?
        (the toplevel window and popup window is decorated with other widget)
Thanks

Eric