QComboBox in CleanLooks style tends to use a scrollbar without having to

The drop down menu from a QComboBox tends to use a scrollbar even when there is a lot of free screen area it could be using. QComboBox::showPopup() will calculate list height as first item height times min of count and max visble items:

listRect.setHeight(itemHeight * qMin(d->maxVisibleItems, count()));

This fails if later items have different height (ie. first item blank, others have text and icons).