setWindowFlags, CustomizeWindowHint, FramelessWindowHint etc does not work correct

int main(int argc, char *argv[]) { 

QApplication mainApp(argc, argv); 

QDialog *dialog = new QDialog(); 
//dialog->setWindowFlags(Qt::Dialog | Qt::FramelessWindowHint); 
//dialog->setWindowFlags(Qt::Dialog | Qt::CustomizeWindowHint); 
dialog->setWindowFlags(Qt::Dialog |Qt::CustomizeWindowHint| Qt::FramelessWindowHint);
dialog->show(); 

return mainApp.exec(); 
}