When does a player lose control of a battlefield with a hidden card after their defending unit dies, and can they still use the hidden card as a reaction?

Ruling: If a unit dies to combat damage during a showdown, control is lost immediately when the showdown ends, and no reactions are allowed from the hidden card. If a unit dies outside of combat (such as from a damage spell), control is lost immediately when no units are present, and the hidden card cannot be used.

Sequence:

• During combat: Lethal damage is assigned, the showdown ends, control is gained/lost, points are gained, then the hidden card is trashed with no reaction window
• Outside combat: When an effect kills the last unit, control is lost immediately and both the unit death and hidden card trashing happen in the same cleanup with no window for reactions

Nuances:

• If a unit dies during the showdown to an effect (not combat damage), control stays until the combat resolves when damage would be dealt
• Deathknell triggers create a special case where chain items can intercede after combat damage but before contested/control is cleared, allowing reactions in that specific window
• Effects like Unchecked Power that kill units outside combat are "checkmate" for hidden cards with no opportunity to use them