Nothing gets topdecked right? The possessed player plays armory, choose a card. The possessing player then gains it instead, and Armory has lost track, so cannot top-deck it.
If so, then fun fact, we have three cards which are functionally identical to Workshop when played while being possessed.
Correct conclusion, wrong reasoning!
Lose-track has absolutely nothing to do with this. Here's what happens:
-Player being possessed plays Armory, and chooses a card to gain.
-Before he gains that card, Possession's "would gain" clause kicks in.
-Instead of Armory gaining a card to the top of your deck, Possession causes the possessor to gain the card.
-The gain that Possession just caused is a normal gain like any other... goes to your discard. The special gain that would have gone on top of the deck has been replaced completely with a normal gain.
This is the same thing with revealing Trader in response to gaining a Curse from Torturer. You will gain a Silver instead of the Curse, but it will go to your discard instead of your hand, because Trader has replaced the "Gain a Curse to your hand" with "Gain a Silver".