Steps to reproduce the issue
1. Run the attached modified Qt example, openglunderqml. It is using QQuickPaintedItem instead of painting to the background of QWindow.
2. Observe the drawing area
Outcome:
QQuickPainterItem's viewport isn't scaled by window()->devicePixelRatio(). The QQuickPaintedItem's texture however has the correct size. Only the bottom left quadrand is rendered on a Retina display.