This only manifests in a very few GTK+ themes -- I've only been able to reproduce it so far with the default GTK+ theme. The issue here is that the code that draws the scrolled window background behind the scrollbar is using gtk_paint_box instead of gtk_paint_shadow. The latter is the correct behavior, which can be observed in gtkscrolledwindow.c in the GTK+ sources.
Created attachment 69649 [details] Patch for this issue
Comment on attachment 69649 [details] Patch for this issue One day we'll get rid of GTK+ themes and the world will be a better place.
Thanks for the review! For posterity, I should correct my previous statement to say that I can only reproduce this issue with the default Ubuntu 9.10 theme.
Comment on attachment 69649 [details] Patch for this issue Clearing flags on attachment: 69649 Committed r69119: <http://trac.webkit.org/changeset/69119>
All reviewed patches have been landed. Closing bug.