When can an opponent activate Stopwatch (Zhonya's Hourglass) during combat, and can the attacker force them to use it before damage assignment to save a different unit?

Ruling: The opponent must activate Stopwatch before damage assignment. Once activated, Stopwatch will save whichever unit would die from the assigned combat damage. There is no reaction window between damage assignment and units dying.

Sequence:

• Showdown happens
• Opponent must activate Stopwatch before damage assignment (if they want to use it)
• Attacker assigns damage
• Stopwatch saves the unit that would die from the assigned damage

Nuances:

• If Stopwatch is activated before damage assignment, the attacker can then choose to assign damage to a different unit, and Stopwatch will save that unit instead
• The opponent cannot wait to see which unit receives damage before deciding to activate Stopwatch
• If Stopwatch is activated and damage is assigned to one unit, the other unit that received no damage will survive anyway