From my solo game 36731238: When there are Coppers in my Exile mat, and Ill-Gotten Gains is used to gain a Copper to hand, then clicking on the Copper I gained is interpreted as affirming that I want to discard my other Coppers from Exile. So the only way to avoid it is to click "Continue" instead of proceeding with my turn by playing the Copper I gained (and only then playing the Copper).
I guess I can see how that carries over from situations where clicking a certain card in hand means choosing that card name for some purpose, but in this context it doesn't seem right at all, because most of the time I just want to play my new Copper but not get any more of them into my deck.
I wouldn't say this is a bug, it actually works as intended. Once you gain a Copper, you need to deal with the question if you want to discard the ones you have in Exile or not. Allowing to click on the card in hand (which has a blue color, not a green color as for playing) is thought of to lessen mouse movements.
But I understand that it may be irritating.
The usual way to deal with those irritating questions for discarding cards like Copper, Curse or Province from Exile is to activate the new "Privince" autoplay. Set it to "Keep bad cards in Exile", and you will not be asked that question on gaining Copper, Curse, Ruins and pure Victory cards (except Feodum and Farmland).
You can set/unset that option globally under Options/Autoplay, or per game by right-clicking on any of the mentioned cards.
Okay, I'll look at that setting, thanks.