[cssom-view] Add script-triggered scroll-snap to list of scrolls affected by 'scroll-behavior'. w3c#4609

Author: fantasai

cssom-view-1/Overview.bs

@@ -1726,7 +1726,8 @@ Canonical Order: per grammar
 </pre>
 
 The 'scroll-behavior' property specifies the scrolling behavior for a <a>scrolling box</a>,
-when scrolling happens due to navigation or CSSOM scrolling APIs.
+when scrolling happens due to navigation, CSSOM scrolling APIs,
+or scroll snapping operations not initiated by the user [[!CSS-SCROLL-SNAP-1]].
 Any other scrolls, e.g. those that are performed by the user, are not affected by this property.
 When this property is specified on the root element, it applies to the <a>viewport</a> instead.