Must a party opposing arbitration on the ground of waiver by litigation conduct prove that it was prejudiced by the other party's waiver?

QUESTIONS PRESENTED This case arises out of an extraordinary course of forum shopping that resulted in an extraordinary incursion on the state courts. Petitioner was sued by his former employer in state court over a contract dispute. When that litigation did not proceed to its liking, the employer reversed course and sought to compel arbitration of the same claims on which it had sued. The state court denied the motion, concluding that the employer waived its right to arbitrate by suing, and that its belated effort to escape the consequences of that choice was blatant forum shopping. Undeterred, the employer turned to the federal courts and persuaded a district court to overlook its waiver, compel arbitration, and enjoin the very state-court proceedings the employer had initiated. The Fifth Circuit affirmed, concluding that even if the employer waived its right to arbitrate, the court would not enforce that waiver because petitioner did not prove that he was prejudiced by being sued in state court. And it affirmed the district court’s refusal to abstain from deciding the very same arbitration issue that the state court had just resolved. The first of those rulings is the product of an acknowledged circuit split that this Court previously granted certiorari to resolve, and the second creates a new one. The questions presented are: 1. Must a party opposing arbitration on the ground of waiver by litigation conduct prove that it was prejudiced by the other party’s waiver? 2. Should a federal court abstain from resolving a request to compel arbitration when a state court has already ruled on that request?