  We derive a constraint on patchy screening of the cosmic microwave background
from inhomogeneous reionization, using off-diagonal TB and TT correlations in
WMAP-7 temperature/polarization data. We interpret this as a constraint on the
rms optical-depth fluctuation \Delta\tau\ as a function of a coherence
multipole Lc. We relate these parameters to a comoving coherence scale, of
bubble size Rc, in a phenomenological model where reionization is instantaneous
but occurs on a crinkly surface, and also to the bubble size in a model of
"Swiss cheese" reionization where bubbles of fixed size are spread over some
range of redshifts. The current WMAP data are still too weak, by several orders
of magnitude, to constrain reasonable models, but forthcoming Planck and future
EPIC data should begin to approach interesting regimes of parameter space. We
also present constraints on the parameter space imposed by the recent results
from the EDGES experiment.
