With the Bluecurve theme, if the scrollbar thumb becomes very small, the three lines on the thumb are drawn outside of the thumb. To reproduce, load a very long document in oowriter.
Its possible that the reason it works in gtk+ is because it has a differen min-size for thumbnail size. I'm not sure. Making it too small doesn't help the user anyway, it just makes it hard to click on.
*** Bug 137019 has been marked as a duplicate of this bug. ***
Created attachment 112720 [details] screenshot Looks alright now with 1.9.90-3 theme bluecurve and 424pages in a writer document.