Does a combat showdown end immediately when there are no units on the battlefield, or does it still require all players to pass?

Ruling: A showdown does not close immediately when there are no units on the battlefield. It still requires all players to pass in a row (instead of starting a chain) before it closes.

Sequence:

• Player 1 attacks Player 2 on Reaver's row
• Player 2 uses reaver's ability (removing units)
• Player 1 passes
• Player 2 can still play a spell with action/reaction speed (like Rebuke)
• Showdown closes only after all players pass in succession