Can you react to a Deathknell trigger with a hidden Zhonya's Hourglass to prevent losing control of it when the unit dies?

Ruling: Yes, you can react to a Deathknell trigger with a hidden Zhonya's Hourglass. Because control does not change until after combat cleanup and all subsequent cleanups, you can resolve a chain from that Deathknell before you lose control of the battlefield.

Sequence:

• Unit dies and Deathknell triggers
• Priority passes to you while Deathknell is in the chain
• You can play the hidden Zhonya's (which has Reaction when hidden) before control changes
• Control change only happens after all cleanup resolves

Nuances:

• This only works if something triggers to start a chain (like Deathknell). Without a trigger, no priority passes between combat damage and control change.
• Hidden cards are not units until played - they're like cards in your hand
• During a showdown, control cannot change until the showdown is over, so you maintain access to hidden cards
• This ruling was under debate and expected to be clarified in version 1.2