How can a player react to bring a unit back to the battlefield after an opponent moves it to base, when the chain closes after both players pass priority?

Ruling: When both players pass in sequence, the chain doesn't fully end - it stops accepting new items and resolves the top item. After that top item resolves, all players regain priority and can react again.

Sequence:

• Player A plays spell targeting unit at battlefield (chain starts)
• Player B reacts with spell to move unit to base
• Both players pass priority in sequence
• Chain stops accepting new items and resolves top item (Player B's move spell)
• Unit moves to base
• Players regain priority and can now react
• Player A can now play spell to move unit back to battlefield
• After passes, original spell resolves with unit as legal target

Nuances:

• "The chain ends" in rule 540.4.b means priority stops passing between players, not that the entire chain is finished
• After each item resolves from the chain, players get priority again before the next item resolves