You can see the thread into the forum:
http://qt-project.org/forums/viewthread/48228/#197447

The problem is when using scroll wheel on a slider or a scrollview, the wheel event now propagate to element below affecting both with a single movement. This become problematic if multiple ScrollView are inside each other or having widget that move position on scroll wheel event.

There's seem to be no way to controls this (wheel event is not pass with the wheelevent just the delta mouvement) and we cannot control the wanted behavior for controls. It's nice to be able to cascade event between widgets, but it would be nice to have control if the event can cascade through some widget or not. Default behavior should be to accept the wheel event and toggle to activate the cascading of that event.

This currently holding us to upgrade our Qt distro (we are sticking with 5.3.0 for now) and cause many problems with our 3D viewport and overlay 2D widgets.