When having wheelEnabled: true in the controls 2 ComboBox to enable scrolling items with mouse wheel, if the index does not change, the wheel events are leaked to the parent.

The ComboBox inherits QML Control which does block wheel event as stated in the documentation:

Note: Care must be taken when enabling wheel events for controls within scrollable items such as Flickable, as the control will consume the events and hence interrupt scrolling of the Flickable.

But despite this, the ComboBox sets the event to not accepted state if the index did not actually change:

event->setAccepted(d->currentIndex != oldIndex);