Can Showstopper be used to move a unit to a battlefield after another unit has already been moved there but before the showdown resolves?

Ruling: No, Showstopper cannot be used once a unit has been moved to a battlefield because the showdown starts immediately, and Showstopper can only be played on your turn, outside of showdown, when the chain is empty.

Sequence:

• When you move a unit to a battlefield, the showdown starts immediately
• Once showdown has started, Showstopper can no longer be played
• Showstopper must be played before moving any units to the battlefield

Nuances:

• Showstopper does not have the "action" or "reaction" keyword, which restricts when it can be played
• It can only be played on your turn when the chain is empty and outside of showdown