When attacking with green Yasuo, can you buff his might after his ability triggers but before damage calculation, or is the ability's damage predetermined at the moment of attack?

Ruling: The ability's damage can be modified after the attack trigger but before damage calculation. You can use reactions to buff Yasuo's might, which will increase the ability's damage.

Nuances:

• When both attacking and defending triggers exist, they go on the initial chain first in that order, and the player controlling the next item on the chain gains priority (which would be the opponent due to the defending trigger)
• In cases with no defending trigger, there is a rules oversight regarding priority that should be fixed in future rules updates
• If Yasuo attacks into a defending trigger like blue Ahri that reduces might, the damage would be reduced accordingly (e.g., to 4 damage assuming no other reactions)