If my opponent uses Retreat to draw his unit back to his hand, then the chain resolves, does he still get a priority to use an action like Stacked Deck or does the combat force end?

The combat does not end simply because your opponent used Retreat.

According to RiftJudge FAQ #976, a showdown (including combat) continues even if there are no opposing units remaining at the battlefield. The combat only ends after both players have passed Focus in succession without starting a new chain.

Here is the breakdown of what happens after your opponent’s Retreat resolves:

1. Chain Resolution: When the chain item (in this case, your opponent's Retreat) finishes resolving, the state returns to Open.
2. Focus/Priority: Per Rule 346, when the chain becomes empty and the turn returns to an Open State during a showdown, Focus passes, and the next player gains both Focus and Priority.
3. Taking Actions: Because the showdown is still ongoing, the player who currently has Focus (and Priority) can play a legally timed card or activate an ability.
   • Since Stacked Deck is an [Action] card, it can be played during a showdown in an Open State (when no chain exists).
   • If it is your opponent's turn to have Focus, they can indeed play Stacked Deck. If it is your turn to have Focus, you may play an action or pass priority to them, at which point they would have the opportunity to play Stacked Deck.

Summary: Using Retreat does not force the combat to end. The showdown remains open, and the players continue to trade priority until both pass in succession. Your opponent can play Stacked Deck if they have Focus and Priority during that Open State.