This is a cosmetic bug that causes practical inconveniences because of the way the “nib” (the position indicator) on scrollbars are drawn.
Take for example the vertical scroll bar.
If one opens a document with lots of lines of text, the vertical scrollbar renders a “nib” that I can click on with the mouse cursor and drag down to see further down the document.
The problem is, even if I drag the nib all the way down so that the end of the document is visible, the “nib” itself is not rendered flush against the end of the scrollbar: There’s about a 16 pixels gap between the end of the scrollbar and where the nib stops.
This causes confusion because at a glance it doesn’t look like I’ve reached the end of the document.
There is a reason for this, but it is hard to describe except to write that it was necessary to be consistent with the Minimap. You can clear the “Show Markers on Vertical Scroll Bar” check box in the Scroll page of configuration properties if you don’t need the Minimap or markers on the scroll bar.