[cssom-view-1] adds notes about user gestures needing to be released

Author: argyleink

cssom-view-1/Overview.bs

@@ -361,7 +361,7 @@ the following steps must be run:
   Otherwise, perform an <a>instant scroll</a> of <var>box</var> to <var>position</var>. After an <a>instant scroll</a> emit the <a event>scrollend</a> event.
 </ol>
 
-Scroll is <dfn lt="scroll completed">completed</dfn> when the scroll position has no more pending updates or translations. This includes smooth or instant mouse wheel scrolling, keyboard scrolling, scroll-snap events, or other APIs and gestures which cause the scroll position to update and possibly interpolate.
+Scroll is <dfn lt="scroll completed">completed</dfn> when the scroll position has no more pending updates or translations and the user has completed their gesture. Scroll position updates include smooth or instant mouse wheel scrolling, keyboard scrolling, scroll-snap events, or other APIs and gestures which cause the scroll position to update and possibly interpolate. User gestures like touch panning or trackpad scrolling aren't complete until pointers or keys have released. 
 
 When a user agent is to perform a <dfn export id=concept-smooth-scroll>smooth scroll</dfn> of a <a>scrolling box</a> <var>box</var> to <var>position</var>,
 it must update the scroll position of <var>box</var> in a user-agent-defined fashion over a user-agent-defined amount of time. When the scroll is