We need to get the offset across documents boundaries, since the scrollframe
might be inside a subdocument. In such cases we were previously computing
a (incorrect) zero offset for the scrollframe, which was throwing off the
margin for the sticky items inside the scrollframe.
MozReview-Commit-ID: AWeQ9ay2cmp
--HG--
extra : rebase_source : 7dd8eda26c482cc359042d79073740524efa792a