what i'm typing in chat gets highlighted & erased

[gov]

I'm on a mac O S X, and when I'm in the chatroom in a private window with someone, what i'm in the middle of typing gets highlighted the second i receive any message from the person i'm chatting with, which means that my next keystroke erases whatever I was just typing.

I don't want this to happen!

Any idea how I can prevent it from happening?

Thank you,
pink girl