If a Showdown is happening at a battlefield and there's a spell (like Stupefy) on the chain targeting a unit, and a player reacts by moving a unit to that battlefield (which would start a new showdown), does the chain from the original showdown close and cause the spell to fizzle?

Ruling: No, the spell does not fizzle. The new showdown only starts after the current showdown finishes, so the chain continues as normal.

Sequence:

• The open showdown continues without attack/defend effects until both players pass focus without playing an action
• The open showdown ends
• If both players still have units at the battlefield (and they weren't killed/moved by spells), nobody conquers the battlefield
• Then a combat showdown starts, and only now do attack/defend effects activate
• The player who moved to the battlefield first is the attacker
• The other player is the defender (even if they don't control the battlefield and can still conquer it if they win)