grabWindow on Mac Retina display captures only partial screen (bottom-left) quadrant

void QuickHelper::grabItem(QQuickItem *item, const QString &filename)
{
    QQuickWindow *w = window();
    QImage img = w->grabWindow();
    while (img.width() * img.height() == 0)
        img = w->grabWindow();
    QQuickItem *rootItem = w->contentItem();
    QRectF rectf = rootItem->mapRectFromItem(item, QRectF(0, 0, item->width(), item->height()));
    QDir docsDir = QDir(QStandardPaths::writableLocation(QStandardPaths::DocumentsLocation));
    pwd.mkdir("output");
    img.save(docsDir.absolutePath()+"/output/" + filename);
    emit imageSaved(item, docsDir.absolutePath()+"/output/" + filename);
}