With the updated core rules on battlefield control, if a hidden unit is flipped after the original solo unit dies mid-combat showdown (due to spell or ability), does the hidden unit get +1 from Mask since it would be alone when designated a defender?

Ruling: Yes, the hidden unit gets the +1 from Mask. With the updated rules, you retain control of the battlefield during a contested showdown even after your units die (as long as death is not from combat damage), so you can flip the hidden unit when you regain priority, and it will be defending alone at that point.

Sequence:

• Opponent initiates combat showdown (battlefield becomes contested)
• Opponent plays action/spell that kills your defending unit
• Priority passes back to you while showdown is still ongoing
• You flip hidden unit (now defending alone)
• Hidden unit gets +1 from Mask bonus

Nuances:

• This only works if units die from spells/abilities during showdown, not from combat damage
• While battlefield is contested, control cannot change
• Outside of combat showdown, you lose control immediately when all units die, so hidden cards would be forced to trash
• The rules regarding contested status and control after combat damage are currently unclear/under debate, with consensus being that intent is you should lose hidden cards after combat damage but before triggers resolve