I am developing a WinUI 3 app that includes TabView. Now I want to create a new window when a tab is dragged out of the current window. But I cannot create another new window because App.m-window is set to the first created MainWindow. The code looks like this:

//App.xaml.cs
public partial class App : Application
{
    public App(){ this.InitializeComponent();}
    public static MainWindow m_window; //This is static so I can call it with "App.m_window"
    protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
    {
        m_window = new(); //m_window is set to the first created window
        m_window.Activate();
    }
}

//MainWindow.xaml.cs
public sealed partial class MainWindow : Window
{
    public TabView CurrentTabview;     //Some other properties
    public MainWindow(){ this.InitializeComponent();}
}

//Page.xaml.cs   This page is navigate by a tabview in MainWindow
public sealed partial class TabPage : Page
{
    public TabPage(){ 
        this.InitializeComponent();
        TabView Item = App.m_window.CurrentTabview; 
        // I have to call current "MainWindow" properties by using "App.m_window"
    }
}

That worked fine. But because I hope to do something like drag the tab out of the current window and automatically create a new window, I couldn’t call App.m_window anymore. Or the page from two different window will call the same MainWindow.

I have tried the solution in How to retrieve the window handle of the current WinUI 3 MainWindow from a page in a frame in a NavigationView control on the MainWindow. But That doesn’t work.

My previous solution of calling MainWindow was making it a static variable in App class. So now I tried with creating a variable ThisWindow in page and set it to App.m_window, so ThisWindow becomes local. Like this, but ThisWindow is “Null”

//Page.xaml.cs
private MainWindow ThisWindow;
void GetCurrentWindow(){ 
    ThisWindow = App.m_window;
    if ( ThisWindow != null)
    {
        Debug.WriteLine("not null");
    }
    else { Debug.WriteLine("Null"); }
    // Output: Null
}

How can I solve it