[cssom-view-1] Define HTMLElement.scrollParent

Define the scrollParent of an element following the discussion and resolution in #1522.

Author: flackr

cssom-view-1/Overview.bs

@@ -1580,6 +1580,7 @@ To <dfn>scroll an element</dfn> (or [=pseudo-element=]) <var>element</var> to <v
 
 <pre class=idl>
 partial interface HTMLElement {
+  readonly attribute Element? scrollParent;
   readonly attribute Element? offsetParent;
   readonly attribute long offsetTop;
   readonly attribute long offsetLeft;
@@ -1588,6 +1589,18 @@ partial interface HTMLElement {
 };
 </pre>
 
+The <dfn attribute for=HTMLElement>scrollParent</dfn> attribute must return the result of running these steps:
+1. If any of the following holds true, return null and terminate this algorithm:
+    * The element does not have an associated [=CSS/box=].
+    * The element is the [=root element=].
+    * The element is <a>the <code>body</code> element</a>.
+    * The element's computed value of the 'position' property is ''position/fixed'' and no ancestor establishes a fixed position [=containing block=].
+1. Let <var>ancestor</var> be the [=containing block=] of the element in the <a>flat tree</a> and repeat these substeps:
+    1. If <var>ancestor</var> is the [=initial containing block=], return the {{Document/scrollingElement}} for the element's document if it is not <a>closed-shadow-hidden</a> from the element, otherwise reutrn null.
+    1. If <var>ancestor</var> is not <a>closed-shadow-hidden</a> from the element, and is a <a>scroll container</a>, terminate this algorithm and return <var>ancestor</var>.
+    1. If the computed value of the 'position' property of <var>ancestor</var> is ''position/fixed'',  and no ancestor establishes a fixed position [=containing block=], terminate this algorithm and return null.
+    1. Let <var>ancestor</var> be the [=containing block=] of <var>ancestor</var> in the <a>flat tree</a>.
+
 The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the result of running these steps:
 
 1. If any of the following holds true return null and terminate this algorithm:
@@ -2075,6 +2088,7 @@ generally not listed.
 * Introduced the {{VisualViewport}} API and related concepts
 * Pinch zoom is now renamed to <a>scale factor</a>
 * Added visibilityProperty, opacityProperty, and contentVisiblityAuto properties for {{Element/checkVisibility()}} method.
+* Added the {{HTMLElement/scrollParent}} attribute.
 
 <h3 id='changes-from-2022-06-22' class=no-num>Changes From 22 June 2022</h3>
 * Adam Argyle moved the <a event>scrollend</a> event from <a href="https://wicg.github.io/overscroll-scrollend-events/">WICG overscroll-scrollend-events</a> to [[CSSOM-VIEW-1]]