If the attacker passes priority, then the defender passes priority, can the attacker then use a reaction or does the showdown end?

Ruling: Once both players pass priority in order during a showdown, the showdown ends. The attacker cannot open a new chain after both players have passed.

Sequence:

• Showdown begins with attacker having focus
• If attacker passes focus, defender gets focus
• If defender then passes focus, the showdown ends immediately

Nuances:

• If there is an initial chain of "when I attack/defend" triggers, that chain resolving keeps focus with the attacker and doesn't count as them passing focus
• Passing focus is a calculated gamble - you might not get focus back