Does a showdown continue to resolve if the last defending unit is moved off the battlefield before the showdown resolves?

Ruling: A showdown only ends when both players pass focus in a row without starting a chain. It does not care if one or both players no longer have units there.

Nuances:

• Players can still play spells, actions, and reactions during the showdown as long as it is still ongoing, even if no units remain on the battlefield