[Qt4-preview-feedback] OSX Assistant broken in Qt 4.5 rc1

[Philippe]

Simple to reproduce: display a large page, for instance the QWidget page,
then scroll down far away with the scroll bar: display gets screw up.

Actually, this was also the case in beta1, but it was so obvious that I
though you were aware of it and it was not worth reporting <g>.

The same happen if you try to view the doc in Qt Creator.

No problem under windows.