During a showdown if attacker pass his Focus and defender plays a reaction during his Focus, does the attacker get the Focus back once the reaction resolves?

Yes, the attacker regains Focus after the reaction resolves.

According to FAQ #5067, when a reaction is played in a showdown and subsequently resolves, priority (and the associated permission to act) returns to the attacker. In a showdown, the state only closes and the showdown ends when both players pass priority in succession without playing any cards.

Here is the step-by-step breakdown of how the focus and priority function in this scenario:

1. Attacker passes Focus/Priority: The attacker chooses not to play an action and passes. Focus and Priority move to the defender.
2. Defender plays a Reaction: Since it is now the defender's turn to act, they play a reaction.
3. Chain resolves: The reaction resolves as a chain item.
4. Focus/Priority returns: Once the chain resolves and the turn returns to an Open State, the right to take discretionary actions (Priority) returns to the attacker. Because Focus is the "home base" for acting in a showdown, the attacker regains their ability to play actions or pass again.

As clarified in FAQ #1602, Focus and priority bounce back and forth between the attacker and defender after each played card resolves. The showdown only concludes when both players pass in direct sequence without adding anything to the chain.

Note: I could not fully verify this ruling against the retrieved rules.