OK, i think i have a better insight in what happens here.
As i see it, the problem is that if you have simultaneously a lot of OnMouseWheel events (produced with a mouse wheel scroll) versus one OnMouseLeave event, the OnMouseLeave will in most cases get pushed out by the OnMouseWheel events and never get called.
If this is true, than some kind of prioritization for the events has to be implemented, i.e. OnMouseLeave should have priority which is greater then OnMouseWheel event.
Does this hold water?