After a chain fully resolves in a showdown, can players play new action/reaction cards before units deal damage to each other, or does the window close?

Ruling: After a chain fully resolves in a showdown, players can play new action/reaction cards. The showdown only ends when both players pass focus without starting a chain.

Sequence:

• Player with focus can start a chain with an action or reaction, or pass focus
• If a chain is started, priority passes back and forth as players add reactions
• Chain resolves last-in-first-out
• Focus passes to the next player in turn order
• That player can start a new chain or pass focus
• Once both players pass focus without starting a chain, the showdown ends
• Special case: Attacker always gets empty focus first for the first chain

Nuances:

• Actions can only start a chain, they cannot be played to an existing chain
• Actions resolve immediately before the opponent gets their showdown window (they don't stack)
• Only reactions (and triggers) stack in chains
• There can be any number of chains in a single showdown