virtual QSize QIconEngine::actualSize(const QSize &size, QIcon::Mode mode, QIcon::State state);

The docs state that it should return logical size. 

The default icon engine for pngs returns device dependent pixels.

How should an implementation find out the DPR to compute the logical size?

IIUC it cant reliably for the window where the icon is used. This should probably be passed by the client code but the api does not allow it.