I don't know if there is an effect in any webkit.org ports, but the offset is passed down to platform specific code, which may not be prepared to get an offset inside a surrogate pair.
Created attachment 97889 [details]
Comment on attachment 97889 [details]
Clearing flags on attachment: 97889
Committed r89322: <http://trac.webkit.org/changeset/89322>
All reviewed patches have been landed. Closing bug.