clearMask does not work in Vista on fullscreen windows.

Setting a mask on a fullscreen window using setMask(), and then clearing it (clearMask()) leaves the mask in place on Windows Vista. A workaround is to call
resize(width()+1, height())
resize(width()-1, height())

after the call to clearMask().

Minimum example attached.

This issue was previously reported in the Nokia support center (when I still had a commercial license) under the title "QWidget::clearMask() does not work on full screen widgets on Windows Vista"

At the time it was claimed that this issue was solved in 4.6, but as of 4.6.2 it is not. Also calling the Win32 function ShowWindow after clearMask does not seem to help.