If I have 2 units at the same location and move one away during showdown, does Mask of Foresight trigger to give the remaining unit +1 for being alone?

Ruling: No. "When I attack/defend" triggers only when a unit becomes an attacker/defender. If you attack with 2 units, they are not alone when they become attackers, so the trigger misses. Moving a unit away afterward does not cause a new trigger because the remaining unit already was an attacker/defender.

Nuances:

• "While" effects (like Wielder of Water or Yi) would apply when the condition becomes true during combat
• "When" effects (like Mask of Foresight) must have their condition met at the moment the unit is assigned as attacker/blocker
• The rules may change in future versions to handle "when I attack/defend" triggers differently