Apple (4:19-cv-06769) and Samsung (4:19-cv-06773) have each filed an analogous complaint in the Northern District of California seeking declaratory judgments of claim preclusion, noninfringement, and invalidity of claims added by IXI IP, LLC and IXI Mobile (R&D) Ltd. (collectively, IXI) to a patent already held invalid. The Patent Trial and Appeal Board (PTAB) cancelled the patent’s claims through a final written decision in an inter partes review (IPR), a decision later affirmed by the Federal Circuit. Litigation over that particular patent nevertheless persisted. Why?