Also in front of the US S Ct - Ripeness/Res Judicata in Takings
The underlying conflict here involves San Francisco's draconian anti-conversion laws, which require huge payments to the City if hotel owners convert their units to condominium or similar uses. Intended to preserve affordable housing in the city, the laws are hugely punitive to anyone who owns property that was used in "single room occupancy" (SRO) long term uses.
This case involves the interplay of ripeness and federal issue preclusion in takings cases. Under the "Williamson County" rule, a state action that creates a taking doesn't violate the federal constitution unless the state refuses to compensate for the taking - which requires presenting the case to the state courts. Other doctrines deal with how you reserve the right to litigate a federal claim in federal court when the same transaction might produce state claims.
The issue here is whether the state courts litigated not only the state's interpretation of the state's taking law (and the availability of compensation), but also the federal issues at the same time. The Ninth Circuit bounced the case, which is now before the Supreme Court on the question.
The problem here is somewhat obscure - ultimately it turns on how do you present a takings case to a federal court so that the federal courts can determine a) the scope of takings protection under the 5th amendment as to new issues, theories or fact patterns; and b) whether federal takings protection exceeds that provided by any particular state. Historically, these core federal questions would be litigated in federal court rather than in state court - it's only because a state court must reject the substantive claim and/or availability of damages to remedy the claim that the taking claims must be presented to a state court. If you can't effectively reserve your federal claims to litigate them in federal court, then the state courts (rather than federal courts) will get to determine what the US Constitution means.
But the federal courts have allowed state courts to develop the scope of the federal constitution in other areas - if San Remo loses here, it will indicate that the future of the takings clause protections will largely be determined by litigating federal questions in state court, with the first contact between the issues and the federal courts being when a state court decision is taken up in a petition for certiorari to the US Supreme Court.