  SS Cyg has long been recognized as the prototype of a group of dwarf novae
that show only outbursts. However, this object has entered a quite anomalous
event in 2021, which at first appeared to be standstill, i.e., an almost
constant luminosity state, observed in Z Cam-type dwarf novae. This unexpected
event gives us a great opportunity to reconsider the nature of standstill in
cataclysmic variables. We have observed this anomalous event and its
forerunner, a gradual and simultaneous increase in the optical and X-ray flux
during quiescence, through many optical telescopes and the X-ray telescopes
NICER and NuSTAR. We have not found any amplification of the orbital hump
during quiescence before the anomalous event, which suggests that the mass
transfer rate did not significantly fluctuate on average. The estimated X-ray
flux was not enough to explain the increment of the optical flux during
quiescence via X-ray irradiation of the disk and the secondary star. It would
be natural to consider that viscosity in the quiescent disk was enhanced before
the anomalous event, which increased mass accretion rates in the disk and
raised not only the optical flux but also the X-ray flux. We suggest that
enhanced viscosity also triggered the standstill-like phenomenon in SS Cyg,
which is considered to be a series of small outbursts. The inner part of the
disk would always stay in the outburst state and only its outer part would be
unstable against the thermal-viscous instability during this phenomenon, which
is consistent with the observed optical color variations. This scenario is in
line with our X-ray spectral analyses which imply that the X-ray emitting inner
accretion flow became hotter than usual and vertically expanded and that it
became denser and was cooled down after the onset of the standstill-like state.
