Hi even yet again,
I found a long discussion thread about the bouncing when entering a long post in the posting textbox at the Simple Machines Forum forum.
The problem is a bug in IE8.
Here's a link to the discussion there if you're interested:http://www.simplemachines.org/community/index.php?topic=309186.0
Some of the people on that thread are saying that if you click the "compatibility mode" button that is located just to the right of the location bar in IE8, the problem will stop. (the button for the compatibility mode looks like a little paper torn across the middle. White Wings put a little screenshot of the button in a post above, but on my screen the image is small enough that I think I need stronger glasses to actually see what it looks like.)
If you're using IE8, try the "compatibility mode" when you're here. Please let me know how it goes.
P.S. In the discussion about this issue at the SMF forum, the problem most people reported just occurs when they put in a post that goes beyond the text box, causing a scroll bar to appear at the right. I'm deliberately jabbering on and on here to make sure that there isn't a problem with a longer post usin IE7.
I can type here without a problem, the scroll bar does not jump back up to the top. That is what has been happening to people, when they type or paste a long message into the post box, the scrollbar keeps jumping up to the top.