If you position the mouse over a QScrollBar and move the mousewheel, you trigger this code in qabstractslider.cpp:

bool QAbstractSliderPrivate::scrollByDelta(Qt::Orientation orientation, Qt::KeyboardModifiers modifiers, int delta)
{
...
        offset_accumulated -= int(offset_accumulated);
        if (stepsToScroll == 0)
            return false;
    }

causes the calling function, QScrollBar::wheelEvent() in qscrollbar.cpp to call event->ignore(), which in turn causes the event to propagate to the parent. But the accumulation code has actually used the event (it accumulated the wheel delta) and event->accept() should be called.

This causes the parent widget to get scroll wheel events that really are intended for the scroll bar.