Does a showdown end immediately if a unit is removed from the battlefield during the showdown?

Ruling: No, once a showdown begins it continues until both players pass consecutively instead of starting a chain. Removing the unit does not end the showdown early.

Sequence:

• A showdown begins when a unit moves to a battlefield (assuming no triggers are responded to that would prevent staging)
• Players may play actions and pass focus back and forth
• The showdown only ends when both players pass in a row without starting a chain
• This is true even if the unit is removed from the battlefield during the showdown

Nuances:

• If a unit has a "when I move" trigger and an opponent responds to that trigger with removal (like Gust), the showdown never begins at all - it will be staged and then unstaged during cleanup after the removal resolves