Can the attacker activate an action card after passing priority, if the defender activates cards and then the chain resolves?

Ruling: Yes, the attacker can activate an action card after the chain resolves. When you pass focus and your opponent activates cards, focus returns to you after the chain resolves, allowing you to play actions.

Sequence:

• Attacker passes focus to defender
• Defender activates reaction/action cards, starting a chain
• Chain resolves completely
• Focus returns to the attacker
• Attacker can now activate action cards
• The showdown only ends when both players pass focus without either starting a chain

Nuances:

• Actions can never be played in response to an ongoing chain - they can only start a chain
• If the attacker had passed focus and the defender also passed without playing anything, the showdown would have ended immediately
• Reactions cannot be played when there is no chain and you don't have priority