child should appear in a secondary group of children.
A typical use of a secondary child is the help button in a dialog.gtkButtonBoxSetChildSecondary(object, child, is.secondary)objectchildwidgetis.secondaryTRUE, the child appears in a secondary group of the
button box.GTK_BUTTONBOX_START, GTK_BUTTONBOX_SPREAD or
GTK_BUTTONBOX_EDGE, and before the other children if the style
is GTK_BUTTONBOX_END. For horizontal button boxes, the definition
of before/after depends on direction of the widget (see
gtkWidgetSetDirection). If the style is GTK_BUTTONBOX_START
or GTK_BUTTONBOX_END, then the secondary children are aligned at
the other end of the button box from the main children. For the
other styles, they appear immediately next to the main children.