Controls if contact and trigger begin/end events for shapes assigned a group are marked as being the first or last event between the two groups involved. This allows the many events produced between two groups of shapes to be reduced to a single begin and end, such as when treating multiple shapes as a single object. The marking is only calculated for shapes assigned a group and only when a begin or end event is produced, so the cost of leaving this enabled is minor. See PhysicsWorld.eventGroupingAllowed