From the productivity power tools extension page:
Enhanced Scrollbar
We’ve been looking into ways that we can
improve the experience of navigating through code files. Our solution
is the source map which has three modes that will allow you to more
easily see the interesting artifacts in your files (edits,
breakpoints, bookmarks, errors, warnings etc) and make it easy for you
to navigate between them. The default mode is the “scroll bar only
mode” which overlays icons onto the standard scrollbar to allow for
viewing of these artifacts. In the source map mode, we’ve replaced the
default scroll bar allow you to click on any item on the scrollbar to
navigate directly to it. This source map mode also provides a preview
of the part of the document as you hover. Finally, we have the
detailed source map mode, which allows you to get a zoom out view of
your entire file. You can switch between any of these modes by
right-clicking on the scroll bar or going to Tools
Options>Productivity Power Tools>Source Map where we have a host of
other options that you can configure.