I think it would be a huge help to the software (and shouldn't take that long to code) if there were two undo buttons.
1. The undo button that currently exists, it just undoes one step. It would be nice if it took you back to the last decision YOU made rather than the last decision ANYONE made (so if you were playing a bot it would actually work for going back to your previous turn, for example). But that's a different feature request.
2. And undo button that lets you undo back to any decision -- essentially something you can click that does the exact same thing as typing "/undo" in the chat does.
I think this second undo button would make the software a lot more usable for a very small cost. Not only does it present the user with one of the most useful features of the software without them having to know about its existence somehow (which is not in the client), but not having to use the keyboard in order to do an arbitrary undo is a huge step in the right direction for usability.