Kordophone/src/application/main-window.vala

using Adw;
using Gtk;

public class MainWindow : Adw.ApplicationWindow
{
    public MainWindow () {
        Object (title: "Kordophone");

        var split_view = new NavigationSplitView ();
        split_view.set_min_sidebar_width (400);
        set_content (split_view);

        var conversation_list_page = new NavigationPage (new ConversationListView (), "Conversations");
        split_view.sidebar = conversation_list_page;

        var message_list_page = new NavigationPage (new MessageListView (new MessageListModel ("123")), "Messages");
        split_view.content = message_list_page;
    }
}