Is there a window to play action or reaction cards in a showdown after damage is assigned and units are dead?

No, there is generally no window to play Action or Reaction cards after combat damage is dealt if no triggered abilities occur.

According to RiftJudge FAQ #492:

• "You can only play cards after combat damage if there is a triggered ability that results from the combat damage. If no triggered ability occurs, the showdown ends immediately."

The Sequence of Events:

1. Combat Damage: Units deal damage to each other.
2. Healing: All units heal (Combat Cleanup).
3. Trigger Check:
   • If triggered abilities exist (e.g., "When I die," "When I take damage"): These are added to the chain. Because a chain now exists, players have a window to play Reaction cards in response to these triggers.
   • If no triggered abilities exist: The showdown ends immediately. There is no priority window to play further cards.

As noted in RiftJudge FAQ #3757, once both players pass priority back-to-back, damage is dealt and the showdown ends with no further window to play cards.