Undo button does not work when discarding cards!
There is NO way to un-select which cards you want to discard, when attacked by a robber. If i click on one by error, there is no way to undo that click, and click on the card next to it instead. If you click something by accident, your fate is sealed!
There are two options at the bottom, a green check box, and a red cross box - they are meant to confirm your decision. The red cross box is permanently greyed out! Then what is the point of having it there?!
Try clicking on the bottom edge of the card. Works for me in iOS version.
The red "X" is to cancel trades, you can't cancel being robbed, but you can easily deselect a card before confirming. Clicking top or bottom of the card adds or subtracts the amount of cards being dicardded or traded.