So I am talking to an officemate and...

I would indeed tell her to back off. Although you were having a chat in a somewhat public space, she can't expect to butt in in such a way and get away with it. I know it may sound cold to some, but if the person irritates you and others, she needs to learn.

I guess there are just people who lack attention and are disrespectful to others. I know a few people back in college who were like that and I usually don't mind them and they eventually stop disturbing me. My advice is not to mind her and don't entertain anything she does, cause that is exactly what she wants.

Hello Chiyosan, First and Foremost, that's rude and have no Manners at all. I think the next time she tries this again, you should be blunt by it and tell her that it's wrong to butt in such discussion.she has no right to do that unless she excuse herself to you two. people like that doesn't have manners at all. I personally find it offending. I know i have done it when i was a kid and was told it's horrible and gore.

That does happens to me sometimes, not just with girls but also a man. I get irritated easily when someone interrupts my conversation with someone else especially if it was a serious one. So what I do is I just honestly tell him or her that they interrupting with some angry look on my face. I do it by joking sometimes, but there was one time one of my good friends in the office was trying to tell me her problem, and I was quite serious listening and trying to understand her. This other girl came in and started tapping the table saying, "what's up guys? (I know she was being friendly lol)" so we just smiled, but then she continued by pulling a chair for her and started complaining about her sleep, the bus, the breakfast without giving us any chance to continue our conversation. We always try to avoid her from having lunch together because she gets the chance to talk all the time, whether we like to listen to not, she doesn't care :)