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

object -- a gtk:header-bar widget
widget -- a gtk:widget custom widget to use for a title

The accessor for the custom-title slot gets or sets the custom title widget of the header bar. The title should help a user identify the current view. This supersedes any title set by the gtk:header-bar-title or gtk:header-bar-subtitle functions. To achieve the same style as the built-in title and subtitle, use the .title and .subtitle style classes.

You should set the custom title to nil, for the header bar title label to be visible again.