Are there multiple windows to play cards during a showdown, and how does priority/focus work between players before combat damage happens?

Ruling: There are multiple opportunities to play cards during a showdown. Combat damage only happens when both players pass focus/priority in succession while the stack (chain) is empty.

Sequence:

• Any "when I move" triggers happen prior to showdown
• When a unit moves to a battlefield, an initial chain happens for "when I attack"/"when I defend" triggers (if any exist)
• After this resolves, the player contesting maintains focus to play an Action if they want
• Priority to play a Reaction passes between players when they pass it (not automatic)
• When they pass with the chain empty, the opponent gets focus automatically and can play an Action
• If they do, focus returns after that chain resolves
• Once both players pass back to back in succession, combat damage happens

Nuances:

• Focus = opportunity to start a chain during a showdown, sometimes with an Action
• Priority = opportunity to add to a chain, must be a Reaction
• The initial chain for attack/defend triggers is conditional - if neither player has such triggers, there is no initial chain and the first opportunity is the attacker's focus in the showdown