Update definition of scrollSource to allow it to be null (#34)

This should fix #11, #12, and #13.

Author: stephenmcgruer

Overview.bs

@@ -27,6 +27,8 @@ urlPrefix: https://w3c.github.io/web-animations/; type: dfn; spec: web-animation
     text: timeline
 urlPrefix: https://drafts.csswg.org/cssom-view/; type: dfn; spec: cssom-view-1
     text: CSS layout box
+urlPrefix: https://html.spec.whatwg.org/multipage/browsers.html; type: dfn; spec: html
+    text: document associated with a window; url: concept-document-window
 </pre>
 <pre class=link-defaults>
 spec:html; type:dfn; for:/; text:browsing context
@@ -341,15 +343,16 @@ behavior under different directionalities and writing modes.
 enum ScrollTimelineAutoKeyword { "auto" };
 
 dictionary ScrollTimelineOptions {
-  Element scrollSource;
+  Element? scrollSource = null;
   ScrollDirection orientation = "block";
   DOMString startScrollOffset = "auto";
   DOMString endScrollOffset = "auto";
   (double or ScrollTimelineAutoKeyword) timeRange = "auto";
   FillMode fill = "none";
 };
 
-[Constructor(optional ScrollTimelineOptions options)]
+[Exposed=Window,
+ Constructor(optional ScrollTimelineOptions options)]
 interface ScrollTimeline : AnimationTimeline {
   readonly attribute Element scrollSource;
   readonly attribute ScrollDirection orientation;
@@ -368,8 +371,23 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 <div class="constructors">
 
 :   <dfn constructor for=ScrollTimeline lt="ScrollTimeline(options)">ScrollTimeline(options)</dfn>
-::  Creates a new {{ScrollTimeline}} object. The attributes of the {{ScrollTimeline}} are 
-    initialized with the respective members of the {{options}} dictionary.
+::  Creates a new {{ScrollTimeline}} object using the following procedure:
+
+    1. Let |timeline| be a new {{ScrollTimeline}} object.
+
+    1. Let |source| be the result corresponding to the first matching condition from below.
+
+        : If the |scrollSource| value of |options| is non-null,
+        :: Let |source| be |scrollSource|
+
+        : Otherwise (|scrollSource| is null):
+        :: Let |source| be the {{scrollingElement}} of the {{Document}} <a lt="document associated with a window">associated</a> with the {{Window}} that is the <a>current global object</a>.
+
+        Note: |source| may still be null after this step, e.g. if the {{Document}} has no {{scrollingElement}}.
+
+    1. Set the {{ScrollTimeline/scrollSource}} of |timeline| to |source|.
+
+    1. Assign the {{ScrollTimeline/orientation}}, {{ScrollTimeline/startScrollOffset}}, {{ScrollTimeline/endScrollOffset}}, {{ScrollTimeline/timeRange}}, and {{ScrollTimeline/fill}} properties of |timeline| to the corresponding value from |options|.
 
 </div>
 
@@ -379,14 +397,6 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 ::  The scrollable element whose scrolling triggers the activation and drives the
     progress of the timeline.
 
-    If this is not specified, the {{scrollingElement}} of the [=active document=]
-    for the current [=browsing context=], as determined at the time the
-    {{ScrollTimeline}} is constructed, is used.
-
-    Note that the {{scrollSource}}'s {{ownerDocument}} may change over the lifetime
-    of the scroll timeline. This does not affect the scroll timeline; the timeline
-    continues to be driven by the scroll progress of the {{scrollSource}}.
-
 :   <dfn attribute for=ScrollTimeline>orientation</dfn>
 ::  Determines the direction of scrolling which triggers the activation and drives
     the progress of the trigger.
@@ -503,8 +513,8 @@ The <dfn>effective time range</dfn> of a {{ScrollTimeline}} is calculated as fol
 The [=current time=] of a {{ScrollTimeline}} is calculated
 as follows:
 
-1.  If {{scrollSource}} does not currently have a [=CSS layout box=], or if its layout box is not
-    a [=scroll container=], return an unresolved time value.
+1.  If {{scrollSource}} is null, does not currently have a [=CSS layout box=], or
+    if its layout box is not a [=scroll container=], return an unresolved time value.
 
 2.  Otherwise, let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
     in the direction specified by {{orientation}}.