c++ - gtkmm：如何使拉伸的小部件正常大小？

Question

从屏幕截图中可以看出，按钮和小部件被拉伸。如何使它们正常大小？这是主要代码。

#include <gtkmm.h>
#include "w.h"

int main ( int argc, char **argv )
{
    auto app = Gtk::Application::create ( argc, argv, "org.gtkmm.test" );

    Gtk::Window window;
    Gtk::Button btn;
    Gtk::Box box(Gtk::ORIENTATION_HORIZONTAL);
    W w;
    btn.set_label ( "test" );
    window.set_default_size ( 200, 200 );
    box.pack_start ( btn, false, false, 0 );
    box.pack_start ( w, false, false, 0 );
    window.add ( box );
    btn.show();
    box.show();
    w.show();

    return app->run ( window );
}