Confirmed.
That seems like a bug, allthough holding CTRL when pressing up/down will work around it... I think it would be better if you could up/down it, like you were holding CTRL in that case, and you can move to different lines of text with Left/Right (like before) and click.
Just remember CTRL!
I do like this editbox though
Edit: If this is the natural way things happen with multiline editboxes, then it's probably best to leave it be... It's just that I (and many other users) have always used UP/DOWN to navigatate previous messages.
