GtkApplicationWindow and its menubar

[Pierre Wieser <pwieser trychlos org>]

Hi all,

I am starting with a small app which will handle documents. 
I want this app has two different menubars :
- a first one when there is no document
- another one when the document is opened.

I have tried to use gtk_application_set_menubar with two
different GMenuModel, but no luck: the GtkApplicationWindow
menubar never changes.

After having searched in the sources, I found that the GtkWidget 
menubar is automatically rebuilt from a GMenu menubar_section.
But the menubar_section itself is initially built in real_realize,
thenonly rebuilt when some gtk-shell settings changes 
(notify::gtk-shell-shows-menubar exactly, but this not relevant here).

So it appears to me that it is just impossible to replace the menubar.
Am I right ? Is it the expected behavior ?
As a side effect, I do not understand the rationale behind
gtk_application_set_menubar: why are we allowed to change the GMenuModel
if the visible GtkWidget is not updated ?

Thanks for some explanations..
Regards
Pierre