Can you play Reactions (like Shen or flip Hidden Units) after your units die to a spell during combat, and maintain control of the battlefield before losing it?

Ruling: During combat (when a battlefield is Contested), you maintain control of the battlefield even after all your units die to a spell, until both players pass focus without taking actions. This allows you to play Reactions like Shen or flip Hidden Units after your units die but before losing control. Outside of combat, you cannot do this - any Reaction would go on the chain before the killing spell resolves, causing the Reaction unit to also die.

Sequence:

• Opponent attacks your battlefield (making it Contested)
• Opponent plays a spell to kill your units
• Spell resolves and kills your units
• Cleanup happens but battlefield remains Contested, so you maintain control
• Focus passes back to you
• You can now play Reactions (Shen, Hidden Units) at the battlefield you still control
• Combat continues until both players pass

Nuances:

• This only works during combat when the battlefield is Contested
• Outside combat, Reactions go on the chain before the killing spell resolves, so they would also be killed
• You can play new Reactions onto the chain after some abilities resolve - there's a priority window after each chain link resolves
• If a Deathknell ability triggers during combat, it creates a priority window where you could React and maintain control