GtkFontChooser::font-activated is documented as-if it was
GtkFontChooserWidget::font-activated.

  /**
   * GtkFontChooserWIDGET::font-activated:
   * @self: the object which received the signal
   * @fontname: the font name
   *
   * Emitted when a font is activated from the WIDGET's list.
   * This usually happens when the user double clicks an item,
   * or an item is selected and the user presses one of the keys
   * Space, Shift+Space, Return or Enter.
    */



In fact, I don't think the interface docs should mention widgets at all,
except as sample implementations.  Nothing prevents, say, a gsettings
based implementation.