Can you play reactions in response to a unit gaining the Attacker designation during combat?

Ruling: You cannot react to the designation itself, but you can react to the "When I attack..." triggers that result from gaining the Attacker designation. The designation is a state change that happens outside the chain, but any attack triggers it causes are placed on the chain, creating a closed state where reactions can be played.

Sequence:

• A unit gains the Attacker designation during combat
• Any "When I attack..." triggers are placed on the chain
• The chain enters a closed state
• Players may now play reaction cards in response to those triggers

Nuances:

• If a unit has no "When I attack..." triggers, gaining the Attacker designation creates no reaction window since nothing enters the chain.
• You are reacting to the triggered abilities on the chain, not to the designation event itself.