gtk:header-bar-title-widget (object)

object -- a gtk:header-bar widget
title -- a gtk:widget title widget for the header bar

The accessor for the title-widget slot gets or sets the title widget of the header bar. When set to nil, the header bar will display the title of the window it is contained in. The title should help a user identify the current view. To achieve the same style as the builtin title, use the title style class. You should set the title widget to nil, for the window title label to be visible again.