Build and run the attached test app on macOS. I've set the plist for this app to use 'low resolution' mode, i.e. with a device pixel ratio of 1.0, even if a retina screen is in use.

Click and drag the label in the top-left corner of the window.

You'll see that the text in the dragged image is fuzzy by comparison with the real thing. It looks as though it's being scaled and resampled in some way. I've attached a screenshot demonstrating the problem.

This is a custom widget with a very simple mouse move handler which creates a pixmap by 'grabbing' a pixmap of the widget. I've also tried this with a custom-drawn pixmap and the same problem occurs, so it's not a problem with QWidget::grab.

If you edit Info.plist and set NSHighResolutionCapable to YES and run it on a retina monitor, it works fine.