Does Volibear's effect trigger when playing Sett (the version that gets +1 power from his on-play buff)?

Ruling: Volibear's effect will not trigger when playing Sett who gains +1 power from his on-play ability, because Sett is not Mighty when played and Volibear checks for Mighty units at that moment, before Sett's buff resolves.

Sequence:

• Sett is played from hand and resolves
• Sett's on-play trigger goes onto the chain to give him a buff
• Volibear's trigger checks right now if a Mighty unit was played (no) and does not go onto the chain
• Sett's on-play trigger resolves, buffing him
• The window for Volibear has passed

Nuances:

• If Sett had 4 power base and was played directly to a battlefield with a buffed unit (making him Mighty via a passive ability), it would trigger Volibear because passive abilities are evaluated on cleanup after Sett resolves, before any triggers are checked
• Passive abilities are evaluated constantly, even without a cleanup