Everytime a carriage return the textarea overlaps. The vertical scroll bar becomes inaccessible. Resizing the windows fixes the problem.
On the next carriage return it happens again.
1/ Install plain XWiki beta 2
2/ Edit a page in Wiki Editor with IE6
3/ Go to the end of a line and type a carriage return
4/ The textarea will resize itself and 20px on the right will hide under the panel zone