How do Action/Reaction spells and When Attacking/When Defending effects interact during Showdown timing, specifically regarding chain resolution order and when Action spells can be played?

Question

Accepted Answer

Ruling: When Attacking triggers go on an "initial chain" first, then When Defending triggers, then the attacker gets priority and focus. This initial chain must fully resolve before any subsequent chains can begin. Players can play Reaction speed spells during the initial chain, but the attacker cannot play Action speed spells while the initial chain is ongoing.

Sequence:
- When Attacking triggers go on the initial chain
- When Defending triggers go on the initial chain
- Attacker gets priority and can play Reaction speed spells
- Opponent can play Reaction speed spells after attacker passes
- Initial chain fully resolves
- Attacker gets focus and priority and can now play Action speed spells
- Subsequent chains can begin

Nuances:
- Passive effects (like wielder of water) resolve immediately without opportunity to react and do not go on the initial chain
- All non-passive triggers go on the initial chain and can be reacted to
- There cannot be multiple chains at the same time