public void set_titlebar (Widget? titlebar)
Sets a custom titlebar for this.
A typical widget used here is HeaderBar, as it provides various features expected of a titlebar while allowing the addition of child widgets to it.
If you set a custom titlebar, GTK+ will do its best to convince the window manager not to put its own titlebar on the window. Depending on the system, this function may not work for a window that is already visible, so you set the titlebar before calling show.
the widget to use as titlebar