Thanks Pavel,
type_hint = Gdk.WindowTypeHint.MENU,
makes my day, thanks a lot :)
window:set_type_hint('POPUP_MENU')
and
local window = Gtk.Window { type_hint = 'POPUP_MENU' }
don't work.
Now, I found also this very informative site : http://www.roojs.com/seed/gir-1.2-gtk-3.0/seed/
It contains an quite exhaustive list of all attributes available to objects : for example, it helped me to fine a way to specify the size of Entries ... that some other sites said it wasn't possible :)
Having a look on Gdk library, I found that Gdk.Window
contains a method named set_decorations() allowing to specify explicitly which decoration we can Put.
But any way to use this methods on a Gtk window ?
Bye