To reproduce, open the testcase, and follow the instructions. Things to notice: when all text is selected, there is a white rect above "amet" and a darker selection shade below it, which is painted over the dots of the "i"s on the next line. When shrinking the selection, if you don't move too fast or pass through "amet", that piece of highlight will remain until you force repainting.
Created attachment 5929 [details] Testcase
In Chromium 4.0.202.0 (23522), Safari 4.0.3 (Win), and Safari 3.2.2 (Win), I can reproduce the white rect and the darker selection shade, but not the highlighting artifact when shrinking the selection.
I can't reproduce the repaint issue anymore, closing the bug.