QMdiArea with TabbedView view renders subwindow on macOS

#include <QApplication>
#include <QMainWindow>
#include <QMdiArea>
#include <QWindow>

#include <QPushButton>

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

QMainWindow mainWindow;

QMdiArea *mdiArea = new QMdiArea(&mainWindow);
mdiArea->setViewMode(QMdiArea::TabbedView);

QWidget *widget1 = new QPushButton("Bar");
mdiArea->addSubWindow(widget1);

QWidget *widget2 = new QPushButton("Foo");
mdiArea->addSubWindow(widget2);

mainWindow.setCentralWidget(mdiArea);
mainWindow.show();

return a.exec();
}