Can you cast an Action spell in showdown after casting a Reaction spell when responding to an opponent's attack?

Ruling: You cannot cast an Action spell while a chain is open. If you cast a Reaction spell in response to an attack, you have opened a chain, which prevents casting Action spells until that chain resolves.

Sequence:

• Player B attacks and passes priority/focus to Player A
• Player A casts a Reaction spell, which opens a chain
• While the chain is open, Player A cannot cast Action spells
• After the chain resolves, the opponent gets focus
• If the opponent passes or the next chain resolves, Player A gets focus again and can then play an Action spell

Nuances:

• You can cast an Action spell during open showdown only if there is no chain currently open
• Simply being in showdown is not enough; the presence or absence of an open chain determines whether Action spells can be cast