[cssom-view-1] adds smooth and instant scroll details, keyboard scroll details and a short definition

Author: Adam Argyle

cssom-view-1/Overview.bs

@@ -337,10 +337,12 @@ the following steps must be run:
    'scroll-behavior' property is ''scroll-behavior/smooth''
    <li><var>behavior</var> is <code>smooth</code>
   </ul>
-  ...then perform a <a>smooth scroll</a> of <var>box</var> to <var>position</var>.
-  Otherwise, perform an <a>instant scroll</a> of <var>box</var> to <var>position</var>.
+  ...then perform a <a>smooth scroll</a> of <var>box</var> to <var>position</var>. Once the position has finished updating, emit the <a>scrollend</a> event.
+  Otherwise, perform an <a>instant scroll</a> of <var>box</var> to <var>position</var>. After an <a>instant scroll</a> emit the <a>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.
+
 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
 <dfn export lt="smooth scroll completed">completed</dfn>, the scroll position of <var>box</var> must be <var>position</var>. The scroll can also
@@ -1709,7 +1711,7 @@ When asked to <dfn export for=Document>run the scroll steps</dfn> for a {{Docume
    <td><dfn event for="Document, Element">scrollend</dfn>
    <td>{{Event}}
    <td>{{Document}}, elements
-   <td>Fired at the {{Document}} or element when the <a>viewport</a> or element has been scrolled, the scroll sequence has ended and any scroll offset changes have been applied.
+   <td>Fired at the {{Document}} or element when scroll is <dfn lt="scroll completed">completed</dfn>: the <a>viewport</a> or element has been scrolled, the scroll sequence has ended and any scroll offset changes have been applied.
 </table>