During a showdown, what types of cards can be played, in what order does priority work, and is there a window to play cards after damage is dealt?

Ruling: During a showdown, players can play actions and reactions, but not normal spells. After both players pass priority back-to-back, damage happens and the showdown ends with no further window to play cards.

Sequence:

• Active player has priority and can play actions or reactions to start a chain
• Opponent can only respond with reactions to that chain
• After the chain resolves, the opponent has priority to play the same types of cards the active player just did
• Priority passes back and forth until both players pass consecutively
• Damage is dealt and the showdown ends

Nuances:

• You cannot "respond" with actions (they don't go on the chain as responses), but both players will have chances to play actions during the priority passing
• It's unclear whether a chain can be started with a reaction (not definitively answered in thread)