WIP:viewport-perform-a-scroll

cssom-view-1/Overview.bs

@@ -444,6 +444,7 @@ the following steps must be run:
 </div>
 
 <div algorithm="perform a scroll of a viewport">
+<!-- @@@:done -->
 
 When a user agent is to <dfn for="viewport" export>perform a scroll</dfn> of a <a>viewport</a> to a given position <var>position</var> and optionally a scroll behavior <var>behavior</var>
 (which is "<code>auto</code>" if omitted) it must perform a coordinated viewport scroll by following these steps:
@@ -465,8 +466,14 @@ When a user agent is to <dfn for="viewport" export>perform a scroll</dfn> of a <
 1. Let <var>element</var> be <var>doc</var>'s root element if there is one, null otherwise.
 1. <a for="/">Perform a scroll</a> of the <a>viewport</a>'s <a>scrolling box</a> to its current scroll position + (<var>layout dx</var>, <var>layout dy</var>) with <var>element</var> as the
     associated element, and <var>behavior</var> as the scroll behavior.
+    Let <var>scrollPromise1</var> be the promise returned from this step.
 1. <a for="/">Perform a scroll</a> of <var>vv</var>'s <a>scrolling box</a> to its current scroll position + (<var>visual dx</var>, <var>visual dy</var>) with <var>element</var> as the associated
     element, and <var>behavior</var> as the scroll behavior.
+    Let <var>scrollPromise2</var> be the promise returned from this step.
+1. If both <var>scrollPromise1</var> and <var>scrollPromise2</var> get resolved with true,
+    return a promise resolved with true.
+    Otherwise (either one gets resolved with false or gets rejected)
+    return a promise resolved with a false.
 
 Note: Conceptually, the visual viewport is scrolled until it "bumps up" against the layout viewport
 edge and then "pushes" the layout viewport by applying the scroll delta to the layout viewport.