I've seen this problem for a while, and am finally reporting the problem (right before I enable backing store, I hope...). The problem doesn't seem to occur as a result of scrolling, but rather from editing activity. As an example, the pane I was typing this window into was occluded on the right side by a web browser. I deleted the last newline in the last paragraph, and a huge portion of that line failed to get redrawn in the process. I also inserted a few characters into the subject line and then deleted them, See the attached picture to see the result.