New version (with Undo!)

[RicardoRix]

The undo feature is strictly forbidden if some else has a turn.

When you are restoring a game situation A from a game situation B:

The Undo action must never change the active player. In other words, if player X clicks on "Undo", it must NOT make player Y active.
If several players did game actions between A and B, you must NEVER provide an undo. In other words, an "Undo" action must NEVER force another player to redo some moves.
No hidden (or private) information must have been revealed between A and B.
No random event with a visible effect must have been triggered between A and B. This includes cards shuffling, dice roll, elements picking, ...

[Ranior]

Huh, well the Terra Mystica implementation as it currently exists does NOT conform to this.

It's slightly tricky though, because sometimes players make decisions in Terra Mystica when it isn't their turn, using the definition of turn as provided by the Terra Mystica game rules.

Basically Player A can do something such as building a structure, and before Player A completes their turn, other players must make this decision about whether to pay a vp for a resource. Player B has this choice, but it isn't actually their turn technically--they can only make this one decision and Player A still has to finish their turn. The undo currently allows Player A to see the decision of Player B on paying a vp for a resource, and Player A can undo that move and choose to do something else as things are presently implemented.

[Een]

You are technically right; to avoid this possibility for abuse with cultists, we should reset the undo when the other player decides to accept power or not. Though, exactly like Ranior said, that's not a great thing to do as it would in some cases deprive a player from correcting a genuine mistake on a complex turn. So the reasoning is that people will play fair, and that those who don't will get flagged and segregated from fair players by the red thumbs and reputation system. If this reasoning is disproved by facts, we may have to change this, but until now (and from Ranior's feedback) is seems that the benefit outweighs the possibility of occasional abuse.

[Raal]

It's not only problem on Cultist.
Any decisions on power leech should not rollback, just it isn't so critical.
I think it's not fair that implementation fault result in player's penalty.
I won't try to abuse this, but It is not absolute cheating because system allows it.
This abuse may take on when intended and also not intended, and have possibility to break tables.

[MoiMagnus]

On some online Chess websites, instead of "Undo", you can ask for a take-back. If the opponent agrees, you undo your turn.
This is a great compromise, and match how we play IRL at boardgame: you can ask for a take-back, and if every other player agrees, you can undo. If at least one player considers that the take-back is an unfair advantage, you can't.

To the devs: Would that kind of mechanics be possible on BGA?
To the players: Do you think that would be a good idea? Or do you think that it will too clunky? Or that it could create some toxicity with some players being pissed-off when their take-back is rejected?

[Sohel]

On some online Chess websites, instead of "Undo", you can ask for a take-back. If the opponent agrees, you undo your turn.
This is a great compromise, and match how we play IRL at boardgame: you can ask for a take-back, and if every other player agrees, you can undo. If at least one player considers that the take-back is an unfair advantage, you can't.

To the devs: Would that kind of mechanics be possible on BGA?
To the players: Do you think that would be a good idea? Or do you think that it will too clunky? Or that it could create some toxicity with some players being pissed-off when their take-back is rejected?

I think this is a great solution.
If a player wants to undo his actions and another player made a decision during his turn (he took or declined power) then all players that made a decision should have to agree to the takeback. Then the abuse is no longer possible.