QGraphicsLinearLayout - strange behaviour with trying to center a widget

I create a horizontal linear layout and add three widgets:

1) QGraphicsWidget with preferredsize 0,0 and sizepolicy QSizePolicy::Preferred,QSizePolicy::Preferred
2) QGraphicsWidget with min and max size of 80,80 and sizepolicy QSizePolicy::Preferred,QSizePolicy::Preferred
3) QGraphicsWidget with preferredsize 0,0 and sizepolicy QSizePolicy::Preferred,QSizePolicy::Preferred

I would expect that the middle widget is centered. However the items 1 and 3 get a size of 0, even if there if room for them to grow. Changing the preferredSize to 1,1 helps in this case, but now consider:

1) QGraphicsWidget with preferredsize 0,0 or 1,1 and sizepolicy QSizePolicy::Preferred,QSizePolicy::Preferred
2) QGraphicsWidget with min and max size of 80,80 and sizepolicy QSizePolicy::Expanding,QSizePolicy::Expanding
3) QGraphicsWidget with preferredsize 0,0 or 1,1 and sizepolicy QSizePolicy::Preferred,QSizePolicy::Preferred

Again I would expect that the middle widget is centered, but it is left aligned with the first and large widget getting a size of 1 only.

Printing out the layout->geometry() in both cases gives a geometry width of 300.

(Test case included, tested with Qt 4.6.3)