Does Yasuo's ability trigger and resolve before the showdown starts, and can Actions be played in response to it?

Ruling: Yasuo's "when I attack" ability triggers as part of the Initial Chain at the start of the showdown, not before it. Only Reactions can be played in response to the Initial Chain; Actions cannot be played until after the Initial Chain resolves and the attacker gains Focus.

Sequence:

• When Yasuo moves to contest a battlefield, he becomes an attacker
• The showdown begins with the Initial Chain
• "When I defend" triggers are added to the Initial Chain first
• "When I attack" triggers (like Yasuo's) are added to the Initial Chain second
• Only Reactions can be played in response to abilities on the Initial Chain
• After the Initial Chain resolves, the attacker gains Focus and can play Actions or Reactions

Nuances:

• Movement triggers (like Traveling Merchant) resolve before the Initial Chain if moving to an enemy-occupied battlefield, and Reactions can be played in response to them
• Action speed cards like Challenge cannot be played during the Initial Chain
• Reaction speed cards like Shakedown can be played in response to Yasuo's ability on the Initial Chain