If I attack and pass priority, then my opponent plays a reaction only, does the combat resolve or does priority pass back to me?

When a reaction is played and resolves during a showdown, priority returns to you (the attacker). The combat does not resolve until both players pass priority consecutively without adding any new items to the chain.

According to FAQ #5067: "When a reaction resolves in a showdown, priority returns to the attacker. Damage only occurs after both players pass priority consecutively without playing anything."

Sequence of Events:

1. You (Attacker) pass priority.
2. Opponent (Defender) plays a reaction.
3. Chain resolves: The reaction executes.
4. Priority returns to you: Because the chain is now empty (or the previous item has resolved), you receive priority again. You may now choose to play an action, a reaction, or pass priority again.

Combat damage only occurs once both players pass priority in succession while the chain is empty. As long as players continue to add items to the chain, the showdown continues.