Progress bar with gtkmm-3.0/c++ doesn't move when copying file

Question

I've made a GUI to copy file easier on Ubuntu 20.04. I'm trying to copy file with a progress bar but it doesn't move. It waits until file copying is ended and goes to 100% suddenly. I used 2 threads : one for copy and one for progress bar.

cout works on console.

Here my code

// copyy iso
std::thread t1([usb, win](){
    std::cout << "Dans le thread t1" << std::endl;
    auto const copyOption = std::filesystem::copy_options::recursive | std::filesystem::copy_options::skip_symlinks;
    std::filesystem::copy(win, usb, copyOption); //copie fichier

});
// progress bar
std::thread t2([usb, f_size_win, &bar](){

    Gtk::ProgressBar *progressBar;
    progressBar = &bar;
    progressBar->set_can_focus(true);
    std::cout << "Dans le thread t2" << std::endl;
    //unsigned long long f_size_usb = 0;
    long double f_size_usb = 0.00;
    double valeur = 0.00;

    std::cout << usb << std::endl;
    std::cout << "Taille de l'iso = " << f_size_win << std::endl;

    do{
        std::this_thread::sleep_for(std::chrono::seconds(1));
        //std::cout << "1 Taille dossier usb = " << f_size_usb << std::endl;
        valeur = f_size_usb / f_size_win;
        std::cout << "Valeur fraction = " << valeur << "\n" << std::flush;
        progressBar->set_fraction(valeur);while (Gtk::Main::events_pending()) Gtk::Main::iteration(false);
        std::cout << "Get fraction = " << progressBar->get_fraction() << std::endl;
        f_size_usb = f_size(usb);
        //std::cout << "2 Taille dossier usb = " << f_size_usb << std::endl;
    }while(valeur < 1);
    //while(f_size_usb < f_size(usb));
});

if(t1.joinable()){
    t1.join();
    std::cout << "thread 1 join" << std::endl;
    std::cout << "FIN DE LA COPIE" << std::endl;
}
if(t2.joinable()){
    t2.join();
    std::cout << "thread 2 join" << std::endl;
}

How to get move progress bar when copying file ?