When Stormbringer kills all enemy units and moves Orange Sivir (Ambitious) onto the battlefield, does Sivir's ability activate and does an attack happen?

Ruling: After Stormbringer deals damage and moves Sivir, units are killed during cleanup, then a showdown starts because Sivir is contesting an empty battlefield. Sivir won't get the attacker designation so won't "conquer after an attack."

Nuances:

• Stormbringer creates a complex interaction involving a loop of combat being staged and unstaged, which has been reported for ORD feedback
• There is ongoing discussion about the exact mechanics of this interaction