Can a player react to a unit moving onto a battlefield before a showdown starts?

Ruling: A standard move without abilities triggering does not allow players to react before the showdown starts. The move triggers a cleanup, stages the showdown, and proceeds directly to the showdown without priority unless something triggers to open a chain.

Sequence:

• Move occurs
• Cleanup is triggered
• Showdown is staged
• Move triggers happen before showdown starts
• If there are attack or defend triggers, players get priority to respond to the initial chain
• If no triggers create an initial chain, no priority is given before showdown begins
• Focus doesn't pass until the showdown is in progress

Nuances:

• Players can only react to a move if something triggers to open a chain (like certain card abilities)
• Move triggers happen before the showdown starts with it being staged
• Players do get priority to respond to the initial chain if one exists, but focus doesn't pass until the showdown is in progress