Message box not centered on resized and maximized QMainWindow

#include <QApplication> 
#include <QMainWindow> 
#include <QMessageBox> 

int main(int argc, char *argv[]) 
{ 
    QApplication a(argc, argv); 
    QMainWindow w; 
    w.resize(800, 600); 
    w.showMaximized(); 
    QMessageBox::question(&w, "Title", "Text", QMessageBox::Yes | QMessageBox::No); 
    return a.exec(); 
}