QWidget, masking child widgets

There is a problem with the masking function(s) when setting the mask on a class that is derived from QAbstractScrollArea. The mask won't mask the viewport which makes the widget able to receive events outside its masked area.
The obvious way to solve it, is to explicitly set a mask on the viewport as well, but that don't strike me as an intuitive way to do it.

I have attached two files, one is an example application showing the problem the second one possible fix/patch for the QWidget source class (Qt 4.6.0).

The problem is more severe when the host-system is OS X, neither setting the mask explicitly on the viewport or applying the patch to Qt (cocoa 4.6.0) seams to work very well.