Can you play an Action spell in response to an opponent's Action spell during a showdown, or can you only play Reaction spells?

Ruling: You cannot play an Action spell in response to an opponent's Action spell during a showdown. You can only respond with Reaction spells because Action speed cannot be played during a closed state (ongoing chain).

Sequence:

• Opponent starts a showdown and plays an Action spell
• You can only respond with Reaction spells
• You must wait for their Action spell to resolve before you can play an Action spell

Nuances:

• Action spells can be played "on your turn, or in showdowns" but only when you have Focus
• When you have Focus, you can start a chain with an Action during a showdown
• Focus is different from priority