If Zhonya's is flipped from hidden during showdown and the opponent responds with Cannon Barrage that would kill the unit, does Zhonya's save the unit or does the unit die before Zhonya's can take effect?

Ruling: Zhonya's will save the unit. When flipped from hidden, Zhonya's as a permanent enters and resolves off the chain immediately with no response window, so its effect is active before Cannon Barrage resolves.

Sequence:

• Player flips Zhonya's from hidden (starts a chain as a reaction)
• Zhonya's immediately resolves off the chain with no response window since it's a permanent
• Priority reverts to opponent who can play Cannon Barrage in response
• Cannon Barrage resolves, dealing damage
• Zhonya's effect saves the unit from dying

Nuances:

• Flipping a card from hidden is a reaction that starts a chain and extends the showdown
• Permanents resolve immediately off the chain with no response window, unlike spells
• Priority reverts to the opponent after the permanent resolves, allowing them to respond to the showdown continuing