The following code example creates a single widget, maximizes it and reports the values returned by QWidget::pos and an QPoint(0,0) mapped to global coordinates which should be the same thing (plus any OS-supplied decoration).

#include <QtGui>

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	
	QWidget w;
	w.show();
	//w.showMaximized();
	
	QPoint p = w.pos();
	qDebug() << "pos" << p.x() << p.y();
	
	p = w.mapToGlobal(QPoint());
	qDebug() << "corrected pos" << p.x() << p.y();
	
	return a.exec();
}

On my Mac it prints the following:
pos 640 410
corrected pos 640 432

Which is reasonable. I would expect 'corrected pos' to return a slightly offset value as it must account for the window decoration.

But if show() is replaced by showMaximized() it prints the following:
pos 640 410
corrected pos 0 44

The 'corrected pos' is as expected (44 is the height of title bar).

But the 640x410 provided by QWidget::pos is now completely wrong.

On MS Windows, the results are almost the same (and identical if the window is frameless).

In our product, we use QWidget::pos and QWidget::geometry to center child windows above the main window. These appear in the wrong position when the parent window is maximized.

The workaround is to use the mapToGlobal(QPoint()) trick.