Is there a priority window to play reaction spells before the showdown if there are no when attacking/defending triggers?

Ruling: No, there is no priority window to play reactions before the showdown begins if there are no triggers. As soon as the move is complete, cleanup happens and combat/showdown begins immediately.

Sequence:

• Move completes
• Cleanup happens
• Combat and showdown begin
• When attacking/defending triggers happen at the beginning of the showdown
• After the initial chain of when attacking/defending triggers resolves and your opponent passes focus, you can play actions (like Rune Prison)

Nuances:

• Move triggers are an exception - they activate and resolve before the showdown begins, creating a window to use reactions and potentially prevent the showdown
• Actions (like Rune Prison) can only be played after you have focus, which is after when attacking/defending triggers resolve
• Stunning a unit after when attacking triggers have resolved will prevent combat damage but not ability damage from when attacking triggers