MainWindow.blp

using Gtk 4.0;

template $ExampleWindow: Gtk.ApplicationWindow {
  title: 'Python - PyGObject - GTK';
  default-width: 683;
  default-height: 384;

  [titlebar]
  Gtk.HeaderBar header_bar {
    [end]
    Gtk.MenuButton {
      icon-name: 'open-menu-symbolic';
      menu-model: primary_menu;
    }
  }

  Gtk.Overlay overlay {
    [overlay]
    Gtk.Box {
      orientation: vertical;
      homogeneous: true;
      margin-top: 12;
      margin-end: 12;
      margin-bottom: 12;
      margin-start: 12;
      spacing: 12;

      Gtk.Button {
        label: 'This Button is below the others';
      }
    }

    [overlay]
    Gtk.Button {
      icon-name: 'go-previous';
      halign: start;
      valign: center;
    }

    [overlay]
    Gtk.Button {
      icon-name: 'go-next';
      halign: end;
      valign: center;
    }
  }
}

menu primary_menu {
  section {
    item {
      label: _('Preferences');
      action: 'app.preferences';
    }
  }
}