- Added nsIScrollableFrame::ScrollMode::SMOOTH_MSD to differentiate
existing smooth scrolls used by keyboard and mousewheel events from the
CSSOM-View scroll-behavior's MSD motion scrolling.
- Implemented ScrollFrameHelper::AsyncSmoothMSDScroll, which takes the role
of ScrollFrameHelper::AsyncScroll when SMOOTH_MSD scrolls are requested.
- Implemented glue code to handle callbacks from AsyncSmoothMSDScroll and
to hand off velocity between the classes when one scroll animation is
interrupted by another.
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