Created attachment 271862 [details]
Screenshot of the Bug
I was able to get into this state once, and I don't know how. Closing and reopening the Inspector fixed it. I was able to drag the thumb freely past the end and it would stop about 20px past the end.
Created attachment 271872 [details]
Comment on attachment 271872 [details]
Clearing flags on attachment: 271872
Committed r196869: <http://trac.webkit.org/changeset/196869>
All reviewed patches have been landed. Closing bug.