The following example program is run on a Mac with retina display:

#include <QApplication>
#include <QWindow>
#include <QScreen>
#include <QPixmap>

int main(int argc, char** argv)
{
    QApplication a(argc, argv);
    QWindow win;
    QPixmap pm = win.screen()->grabWindow(0, 0, 0, 20, 30);
    pm.save("/tmp/a.png");
}

The saved image is of size 40x60 pixels which seems to be correct. But the displayed area is not the retina content, but rather the content of the logical coordinates 40x60 instead of the logcial coordinates 20x30.