When an opponent uses an effect to move one of my units to a battlefield they occupy (starting a showdown), who gains focus and priority?

Ruling: The player who owns the unit that was moved gains focus. The moved unit applies contested status and becomes the attacker, giving its owner focus (assuming the unit has no "when I attack" abilities).

Sequence:

• Opponent moves your unit to their occupied battlefield
• Your unit applies contested status to the battlefield
• You gain focus as the attacker
• Showdown proceeds with you having focus

Nuances:

• This works the same whether you move your own unit or your opponent moves it - what matters is who owns the unit applying contested
• If the unit has "when I attack" abilities, those would trigger and affect focus
• If moved to an open (unoccupied) battlefield, focus and priority work the same way, but the unit is not considered an attacker unless there is actual combat