Background:

Spatnav's algorithm can either focus, scroll or do nothing (in that order). If spatnav didn't find anything to focus, it will try scroll the current container.

Motivation: 

Smooth scrolling would make algorithm's choice (to scroll) more apparent. A smooth scroll, compared to spatnav's current "X pixels at once"-scrolling, is probably easier to notice for end users.

Regular arrow-key scrolling (when not using --enable-spatial-navigation) already uses smooth scrolling by default.