I've had the same problem for quite some time, too; several months, as a matter of fact. If the message goes beyond the bottom of the box, the box does not "advance." However, if I keep typing, inbetween keystrokes, the box "jumps" back up, so while I'm typing (if it's fast enough), it doesn't do it, but if I go slower, the box jumps up-and-down-up-and-down-up-and-down. I haven't mentioned it before because, while it can be a bit irksome, it doesn't prevent me from blabbering on and on when I get long-winded.
Gerard