Nieves v. Bartlett, 17–1174 (May 28, 2019): Probable cause to arrest defeats a First Amendment retaliation claim except where there otherwise would not have been an arrest (maybe a really hard standard to meet).
SCOTUSBlog: Opinion analysis: The First Amendment, probable cause and questions left unanswered by Howard M. Wasserman :
On Tuesday, in Nieves v. Bartlett, a majority finally agreed on a standard for how probable cause affects a civil damages action for First Amendment retaliatory arrest under 42 U.S.C. § 1983: A plaintiff must show the absence of probable cause to arrest as an element of the claim and the presence of probable cause will defeat most claims, unless a plaintiff presents “objective evidence that he was arrested when otherwise similarly situated individuals not engaged in the same sort of protected speech had not been.” But the decision produced five opinions and left questions for the lower courts to resolve, hopefully “commonsensically.”
This was originally not listed in pending cases because, reading the briefs, it was too hard to figure out where it would go and whether it would end up a First or Fourth Amendment case.