frameless QMessageBox: Buttons not usable via Touchscreen

#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QMessageBox box;
    box.setText("box");
    box.setStandardButtons(QMessageBox::Ok | QMessageBox::Cancel);
    box.setWindowFlags(Qt::FramelessWindowHint);
    box.exec();
}
MainWindow::~MainWindow()
{
    delete ui;
}