A GtkVBox corresponding to the "message area" in the message_dialog. This is the box where the dialog's primary and secondary labels are packed. You can add your own extra content to that box and it will appear below those labels, on the right side of the dialog's image (or on the left for right-to-left languages). See gtk_dialog_get_content_area() for the corresponding function in the parent GtkDialog.
Since 2.22