compatibility - problem IE8

If I write an answer (in my forum or here on phpbb.com) with the new InternetExplorer 8.0 (no beta, no rc) I get problems with long (many lines) answers.
From a certain numer of lines, I can not see waht I write, because the answer-filed always jumps/scrolls up with each sign I type ... I hope you understand waht I mean.

Try it: Copy some sentences in an topic-anser ... copy again and again ... you will see, what I mean.

Edit: If I use the compatibility-modus of the IE8 there is no problem.