Qt::MouseEventSource QMouseEvent::source()
returns Qt::MouseEventNotSynthesized for mouse events, generated
by OS from Microsoft Pen input.

I'd expect it to return Qt::MouseEventSynthesizedBySystem

Note: I did see this

Note: Many platforms provide no such information. On such platforms Qt::MouseEventNotSynthesized is returned always.

And according to this Windows provides that info
https://msdn.microsoft.com/en-us/library/windows/desktop/ms703320(v=vs.85).aspx