[![Only button with label has linked style][1]][1]

Only button with label has linked style.

    var button_box = new Gtk.Box (Gtk.Orientation.HORIZONTAL, 0);
    button_box.get_style_context ().add_class (Gtk.STYLE_CLASS_LINKED);

    info_button = new Gtk.Button.from_icon_name ("dialog-information-symbolic", Gtk.IconSize.LARGE_TOOLBAR);
    button_box.add (info_button);

    button_with_label = new Gtk.Button.with_label (_("Test"));
    button_box.add (button_with_label);

    header_bar.pack_start (button_box);


  [1]: https://i.sstatic.net/wUOxe.png