Does a cleanup happen immediately after spells resolve outside of combat, causing a unit to die when it reaches 0 or less health?

Ruling: Yes, cleanups happen whenever the board state changes, including when items on the chain resolve. In the example given, after Stupefy resolves, a cleanup occurs and Kaisa dies.

Sequence:

• Hextech Ray resolves, dealing 3 damage to the 4 health unit
• Cleanup occurs
• Stupefy resolves, reducing might by 1
• Cleanup occurs (item removed from chain)
• Unit now has lethal damage and dies during this cleanup

Nuances:

• Cleanups cannot happen in the middle of abilities resolving
• Cleanups happen frequently throughout a turn (potentially dozens of times)
• Cleanups occur when items are removed from the chain (resolve), not only when transitioning between Open/Closed states
• Players typically don't need to think about cleanups explicitly as they handle natural consequences of game actions