The Diff window in Sourcetree doesn't have its own scrollbar, and the current implementation with a scrollbar at the bottom of each hunk is not user friendly at all, as you have to scroll to the bottom of the hunk, move the scrollbar, then scroll back up to the part of the text you want to see. There have been dozens of Horizontal Scrollbar bug reports, so this is definitely something that needs improvement.

Suggestion:

Please move the scrollbar to the scrollview itself, instead each of the hunks, so it will ALWAYS be accessible without having to scroll down and back up.  Also, if you make the scrollbar's scroll-distance equal to the longest window of all of the hunks being displayed, then a single scrollbar can work for everything.  This would be a much nicer user experience.

Thanks for all the work that has gone into Sourcetree.  It has a much cleaner interface than Git Extensions by far.