I have a simple Undo policy: if an action resulted in ANY new information being made available to the opponent, I think it is very inappropriate to Undo that. That information could be: saw a new card from their deck, or found out whether I was going to React, or even found out that the game was giving me an *opportunity* to React - anything like that.
But in any other case but that I am always fine with granting it, even multiple steps. In fact, it feels really bad/unfriendly not to, especially if I'm winning.
I would *really* like to just set that as my preference, but the bigger issue is that it is though for me to manually puzzle out whether the opponent is trying to do an illegitimate Undo like this. Can something be done to help this situation?