  We consider spin polarization of massless fermions in a shear flow, whose
complete contributions contain magnetization current and side-jump current
known from collisional chiral kinetic theory. We argue that the side-jump
current adopts interpretation of displacement current. We explicitly determine
the displacement current contribution in the steady state reached in shear flow
for a QED plasma. We find the displacement contribution enhances the
magnetization contribution at small and large momenta, but leads to a
suppression effect at intermediate momenta. Major differences from previous
studies on collisional effect are: (i) the fermions are in the same steady
state as the medium rather than being probes; (ii) Compton scattering and pair
annihilation are also included in addition to the Coulomb scattering considered
before.
