If my opponent plays Downwell and returns all units and gear (including Treasure Trove) to hand, can they react with their hidden unit using Treasure Trove's trigger to maintain battlefield control?

Ruling: Yes. When Downwell resolves and returns Treasure Trove to hand, Treasure Trove's "When this leaves the board" trigger is placed on the chain as a pending item. Because the chain is not empty, the game is not in an open state, so your opponent does not lose control of the battlefield during cleanup. When Treasure Trove's trigger finalizes and creates a reaction window, your opponent can play their hidden unit to maintain battlefield control.

Sequence:

• Downwell resolves, returning all units and gear to hand
• Treasure Trove's trigger becomes a pending chain item
• Downwell finishes resolving
• Cleanup occurs, but battlefield control is not lost because the turn is not in an open state (chain is active)
• Treasure Trove's trigger finalizes, creating a reaction window
• Opponent can play their hidden unit during this window

Nuances:

• Rule 187.4.c specifies that battlefield control is only lost during cleanup if the turn is in an open state
• If Downwell resolved with no triggers generated, the opponent would lose control and the hidden card would be discarded