Scrollbars are made to be slightly smaller than the region that they are in, but that margin is quite small at 0.1 x U.widget_unit. Unfortunately we have many places where that is too close to a corner and therefore overlaps with the splitter/join areas at the corners. It can be quite a shock to try to move a scroller only to split the region without warning.
This patch just changes that margin so that the ends (top and bottom for verticals, left and right for horizontals) are pulled in a total of .4 of a U.widget_unit. rather than .1 so that no interference can occur at the corners.