Can reaction speed spells or hidden cards be played during an opponent's turn without the opponent initiating anything?

Ruling: No, reaction speed spells and hidden cards cannot be played without a priority window. Reactions always require either a showdown or something on the chain to be used.

Nuances:

• This restriction applies on your own turn as well (though you gain priority by default during a neutral open state on your turn)
• You cannot use reactions at the end of your opponent's turn without a triggering event
• There must be either a showdown or something on the chain to enable reactions