During a showdown (closed state), can you cast a spell to target units on a different battlefield?

Ruling: Yes, you can cast spells targeting a different battlefield while a showdown is happening at another battlefield. For example, if a showdown is occurring at battlefield A, you can play Void Seeker targeting battlefield B.

Nuances:

• This is commonly used to prevent hold points or punish poor sequencing
• Once the showdown resolves, the player would not be able to cast that spell since it's at action speed
• Hidden cards are an exception to this rule