  Narrow transient emission lines (flash-ionization features) in early
supernova (SN) spectra trace the presence of circumstellar material (CSM)
around the massive progenitor stars of core-collapse SNe. The lines disappear
within days after the SN explosion, suggesting that this material is spatially
confined, and originates from enhanced mass loss shortly (months to a few
years) prior to explosion. We performed a systematic survey of H-rich (Type II)
SNe discovered within less than two days from explosion during the first phase
of the Zwicky Transient Facility (ZTF) survey (2018-2020), finding thirty
events for which a first spectrum was obtained within $< 2$ days from
explosion. The measured fraction of events showing flash ionisation features
($>36\%$ at $95\%$ confidence level) confirms that elevated mass loss in
massive stars prior to SN explosion is common. We find that SNe II showing
flash ionisation features are not significantly brighter, nor bluer, nor more
slowly rising than those without. This implies that CSM interaction does not
contribute significantly to their early continuum emission, and that the CSM is
likely optically thin. We measured the persistence duration of flash ionisation
emission and find that most SNe show flash features for $\approx 5 $ days.
Rarer events, with persistence timescales $>10$ days, are brighter and rise
longer, suggesting these may be intermediate between regular SNe II and
strongly-interacting SNe IIn.
