QWidget::mapToGlobal() and mapFromGlobal not working in some conditions

when a qwidget is created and its geometry is set programmatically, mapToGlobal and mapfromglobal return incorrect values.
the attached test case shows the problem.
with it i get the following output:

QRect(0,0 640x320) QPoint(0,0) QPoint(0,0)
QRect(100,100 100x100) QPoint(104,123) QPoint(0,0)

while the second line should be:
QRect(100,100 100x100) QPoint(100,100) QPoint(-100,-100)