A minimal reproducer that is adapted from "diagram scene" example is attached. Build and run, then scroll around, one will see "tearing":

If the screen is scaled, e.g. by 125% or 150%.

That is due to

view->setCacheMode(QGraphicsView::CacheBackground);

Without caching, the problem is gone.

Plus, "setDevicePixelRatio" does not help here. Tearing still happens no matter if QPixmap knows the correct pixel ratio or not.

What original reporter suspects:
The fundamental issue is that the cached background is a pixmap and it can only be shifted full pixels, where as the backgrond drawing happens to subpixel accuracy to the same backbuffer.