My opponent attacks into my solo unit at Star Spring. I have a Mask of Foresight in base. I ambush and play Rengar to Star Spring and trigger Star Spring to move my other unit back to base. Would the Mask of Foresight still give my Rengar +1 might for defending alone?

No, Mask of Foresight will not give your Rengar +1 Might in this scenario.

According to RiftJudge FAQ #4433 and FAQ #6385, the condition for Mask of Foresight ("when I attack/defend alone") is checked exclusively at the moment units are initially designated as attackers or defenders at the start of a showdown.

Why it does not trigger:

1. Initial Check: When the opponent attacked Star Spring, your solo unit was designated as the defender. At that specific moment, Mask of Foresight checked for the "defends alone" condition. If it was indeed alone, it would have triggered then.
2. No Re-designation: Moving units in or out of the battlefield after that initial designation—such as ambushing Rengar in or using Star Spring to move your original unit back to base—does not cause the remaining units to be re-designated as defenders.
3. Static Condition: Because the trigger condition was not met at the moment of designation (or has already been evaluated), changing the number of defenders later does not cause the Mask to trigger or grant a bonus to the remaining unit.

Summary of Timing:

• Mask of Foresight is not a continuous effect; it is a triggered ability that looks for a specific state only at the moment designations are applied during the combat's initial chain.
• Once that initial designation moment passes, adding or removing units does not create new opportunities for Mask of Foresight to trigger for that same combat.