I'm sorry to be the bad ambassador but I need to report another thing. I'm not sure this is really a bug or if it's just my browser (Chrome), but I rebooted and nothing changed. I have a MacBook Pro with Lion installed. Chrome is version 18.0.1025.165. If there is a need of some data I forgot, please comment below.
I took a quick vid and made a gif that reproduces exactly what I see. If you want to try reproducing it, you should know that I noticed that it doesn't happen always so don't get discouraged at the first try. Another thing is that it triggers only if you scroll down the chat even if you reached the bottom, when you do, scroll down another time. This is what I see:
Yes, this is very annoying. :(