Regression(?) in undo

Previously, the undo stack was reset when you changed the deck you reviewed.
Now, if you review the last card from a deck (e.g. German), then select a distinct deck (Japanese) and press Ctrl+z, not only the last card from the last deck is un-reviewed, but it’ll show up.
In that case, you’d have selected Japanese and suddenly get a German card to review. And even if you press 1/again, you won’t see it again, so the indication 1 minute is actually false

I’m having trouble following you. Deck changing is an undoable operation now, so when you undo after changing the deck, it should say “Change Deck undone”, and present the next due card from the previous deck.

Indeed, I now see “Change deck undone”. I guess I just missed the fact that the message was more specific than “undone”.
may I suggest indicating the name of the deck that is selected again somewhere, so that it is clear where the user is back.

Only a message is exposed to the frontend, so it can’t currently take specific actions based on which operation was undone.