I'd guess it has to be the front-mech shifter (as it has no friction) or a rear one with the friction removed somehow.
A Front shifter still has friction to counteract the spring in the derailleur. Do you mean no indexing, and with the friction relieved so the brake's spring can pull the cable back?
A Front shifter still has friction to counteract the spring in the derailleur. Do you mean no indexing, and with the friction relieved so the brake's spring can pull the cable back?