gtk:dialog-use-header-bar (object)

object -- a gtk:dialog widget
setting -- true if the dialog uses a header bar

Accessor of the use-header-bar slot of the gtk:dialog class. True if the dialog uses a header bar for action buttons instead of the action area. For technical reasons, this property is declared as an integer property, use the value 1 for true or -1 for false.

Builtin gtk:dialog subclasses such as the gtk:color-chooser-dialog widget set the use-header-bar property according to platform conventions, using the gtk-dialogs-use-header setting.

Here is how you can achieve the same:

(let* ((settings (gtk:settings-default))
       (setting (g:object-property settings "gtk-dialogs-use-header"))
       (dialog (make-instance 'gtk:dialog
                              :use-header-bar setting)))
  ... )  

The gtk:dialog implementation is deprecated since 4.10. Use the gtk:window widget instead.