I know that no player is ever obliged to grant an undo, even if no information is gained; some players refuse ever to ask for, or grant, any undo on principle, and that's fine. I just ran into a bit of an odd situation, though, that seems at least bad manners if not bad sportsmanship.
Duplicate, Border Village, and Swamp Hag were in play; I had a Duplicate on my Tavern mat and bought a Border Village while Swamp Hag was in effect. I knew I would get a Curse, of course, but wanted to Duplicate the Border Village (and gain a second cheaper card); however, I accidentally called the Duplicate right after gaining the Curse (whoops)!
Not being a Stoic myself, I requested an undo, and put a note in the message box apologizing and explaining why I wanted it. My opponent merely sat there; he neither granted nor denied the undo, nor did he reply to my message. I posted another message saying that he was within his rights to refuse, but he should at least be willing to do so explicitly. He continued to ignore it, and I timed out and was auto-resigned shortly afterwards.
I admit that I knew that this would happen eventually; if he had denied the undo under the circumstances I would have been a bit annoyed, but I would have finished the game. I was determined to make him explicitly deny the request if he wouldn't grant it, though.
Is it reasonable to expect that one's opponent will either grant or deny an undo (perhaps after first asking why one wanted it), or am I just being silly?