Black artifacts when scrolling QTextEdit on Mac

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QTextEdit>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    QTextEdit *textEdit = new QTextEdit();
    for (int i(0); i < 100; i++)
        textEdit->setPlainText(textEdit->toPlainText() + QString::number(i) + "abc\n");
    w.setCentralWidget(textEdit);
    w.show();
    return a.exec();
}

// Comment out the following line of code:
//qt_mac_clearDirtyOnWidgetInsideDrawWidget = true;
qwidgetPrivate->drawWidget(qwidget, qrgn, QPoint(), flags, 0);
qt_mac_clearDirtyOnWidgetInsideDrawWidget = false;