MainWindow.blp

using Gtk 4.0;

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

  [titlebar]
  Gtk.HeaderBar {
    title-widget: Gtk.StackSwitcher {
      stack: stack;
    };

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

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

    Gtk.Stack stack {
      vexpand: true;

      Gtk.StackPage {
        name: 'page1';
        title: 'Python - PyGObject - GTK';

        child: Gtk.Box {
          orientation: vertical;
          spacing: 6;

          Gtk.Button {
            label: 'Button 01';
          }

          Gtk.Button {
            label: 'Button 02';
          }

          Gtk.Button {
            label: 'Button 03';
          }
        };
      }

      Gtk.StackPage {
        name: 'page2';
        title: 'Python - PyGObject - GTK';

        child: Gtk.Box {
          orientation: vertical;
          spacing: 6;

          Gtk.Label {
            label: 'Line 01';
          }

          Gtk.Label {
            label: 'Line 02';
          }

          Gtk.Label {
            label: 'Line 03';
          }
        };
      }
    }
  }
}

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