The case is In the Matter of D.K.; it was decided by the Wisconsin Supreme Court in 2020, but I hadn't noticed it then, and just learned about it because of a new decision that cited it. From Justice Annette Kingsland Ziegler's majority opinion:
[T]he County had to prove by clear and convincing evidence that D.K. was mentally ill, a proper subject for commitment, and dangerous. The circuit court concluded that Winnebago County met its burden of proof, ordered D.K.'s involuntary commitment for six months, and ordered involuntary medication and treatment. The court of appeals affirmed. It concluded that D.K.'s threats and plans to strangle police officers and kill other people established a "'reasonable fear … of serious physical harm' under § 51.20(1)(a)2.b.," and, therefore, "the circuit court's dangerousness determination… was supported by the evidence." …
The order had expired by the time it came up to the Wisconsin Supreme Court, so normally the mootness doctrine would keep the court from review the order. But the court held that "D.K.'s commitment is not a moot issue because it still subjects him to a firearms ban":
