What comes first: A Berserker Barbarian's Retaliation attack or a Concentration Check?

Question

Let us assume there is a Berserker Barbarian who is currently concentrating on a smite spell such as wrathful smite. Because they are concentrating on a spell the following applies:

Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration...

Notably this does not require their action, bonus action, or reaction at all. However, the Barbarian also has the Retaliation feature which states:

When you take damage from a creature that is within 5 feet of you, you can use your reaction to make a melee weapon attack against that creature.

This does require your reaction but it has the same trigger as a concentration check (taking damage).

Xanathar's Guide to Everything has an optional rule (page 77) on "Simultaneous Effects" which states:

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

I am unsure whether these still count as happening at the same time when one of them requires a reaction and the other does not.
Do these effects occur in a specific order or does the active player/GM decide their order?

Answer 1

The concentration check occurs first

When you take damage, you make the Constitution saving throw to maintain concentration. This is a concurrent event. In contrast, reactions always take place after the trigger unless a timing is specified otherwise (Dungeon Master's Guide):

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes

This means you take the damage, which is concurrent with the concentration check, and then have the option to Retaliate.