Using these lines to capture the contents of an external screen (QScreen)

const QRect geometry = screen->geometry();
pixmap = screen->grabWindow(desktop->winId(),
         geometry.x(), geometry.y(),
         geometry.width(), geometry.height()
         );

the geometry here is e.g. QRect(-960,0 960x540) for an external monitor positioned to the left of the main screen.

If I move the external monitor to the right of the main screen under System Preferences -> Displays, the capture works as expected. It also works as expected when making the external monitor primary, regardless of which side it's on. Internal monitor on left hand side on (now primary) external monitor gives a geometry of QRect(-1366,0 1366x768)

Possibly related: QTBUG-44422