An op-ed entitled A Major Church-State Ruling That Shouldn't Have Happened claims that
In Trinity v. Comer, there was no remaining dispute between the actual parties
As I understand it, courts have no power to seek out cases or issues to rule on. A party with standing must bring the case to the court. (I've usually heard this in response to rhetoric about "activist judges.") So either my understanding has been wrong, or there's more to the story of Trinity v. Comer.
If there was no remaining dispute, why did SCOTUS hear this case?