QWidget::render() does not perform correctly on retina screen

When we want to support hidpi raster content the use of QWidget::render() is not correct and produce completely transparent background in some area.
The transparent area color is not random and correspond to the QImage::fill() color given meaning that some area are not drawn at all.

  const qreal ratio = widget->window()->devicePixelRatio();
  QImage img(widget->size() * ratio, QImage::Format_ARGB32);
  img.setDevicePixelRatio(ratio);
  img.fill(Qt::transparent);
  widget->render(&img);

As you can notice too, style are wrongly drawn (bottom line in each QToolBox sections).