[css-scroll-anchoring-1] Add more details to anchor node selection (#3476)

* [css-scroll-anchoring-1] Fix link-defaults for content-area and css2 properties

* [css-scroll-anchoring-1] Make `position: sticky` an excluded subtree

`position: sticky` nodes aren't suitable anchor candidates as they may
become stuck in the future, which causes them to behave similarly to
`position: fixed`.

* [css-scroll-anchoring-1] Add condition to not select scroll anchor when not scrolled from origin

* [css-scroll-anchoring-1] Move and expand section on anchor node invalidation

This commit moves the existing paragraph about anchor node invalidation into
a subsection, and adds precise conditions upon when an anchor node is considered
invalidated.

* Apply suggestion from @tabatkins

---------

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: eqrion

css-scroll-anchoring-1/Overview.bs

@@ -21,6 +21,7 @@ Abstract: This spec also proposes an API for web developers to opt-out of this b
 
 <pre class=link-defaults>
 spec:css-box; type:dfn; text:content area
+spec:cssom-view-1; type:dfn; text:scrolling area
 spec:css2;
 	type:dfn; text:line box
 spec:css-sizing-3;
@@ -139,17 +140,19 @@ non-atomic inline element as the priority candidate.
 	The <dfn id="anchoring-algorithm">anchor node selection algorithm</dfn>
 	for a scrolling box |S| is as follows:
 
-	1. If |S| is associated with an element
+	1. If |S| is generated by an element
 		whose computed value of the 'overflow-anchor' property is ''overflow-anchor/none'',
 		then do not select an anchor node for |S|.
-	2. Otherwise, for each <a>priority candidate</a> |PC| in order specified,
+	2. If |S| is not scrolled away from the origin of its <a>scrolling area</a> in its
+		<a>block flow direction</a>, then do not select an anchor node for |S|.
+	3. Otherwise, for each <a>priority candidate</a> |PC| in order specified,
 		check if |PC| is a <a>viable candidate</a> in |S|.
 		If so, perform the <a>candidate examination algorithm</a> for |PC| in |S| and terminate.
 
 		Note: Since the priority candidate itself is checked to be viable, the <a>candidate examination algorithm</a>
 		is guaranteed to select at least the candidate node itself.
 
-	3. Otherwise, for each DOM child |N| of the element or document associated with |S|,
+	4. Otherwise, for each DOM child |N| of the element or document associated with |S|,
 		perform the <a>candidate examination algorithm</a> for |N| in |S|,
 		and terminate if it selects an anchor node.
 
@@ -182,17 +185,13 @@ non-atomic inline element as the priority candidate.
 		content to shift without triggering any scroll anchoring adjustment.
 </div>
 
-Conceptually, a new anchor node is computed for every scrolling box
-whenever the scroll position of any scrolling box changes.
-(As a performance optimization,
-the implementation may wait until the anchor node is needed before computing it.)
-
 <div algorithm>
 	A DOM node |N| is an <dfn>excluded subtree</dfn>
 	if it is an element and any of the following conditions holds:
 
 	* |N|’s computed value of the 'display' property is ''display/none''.
 	* |N|’s computed value of the 'position' property is ''position/fixed''.
+	* |N|’s computed value of the 'position' property is ''position/sticky''.
 	* |N|’s computed value of the 'position' property is ''position/absolute''
 		and |N|’s <a>containing block</a> is an ancestor of the scrolling box.
 	* |N|’s computed value of the 'overflow-anchor' property is ''overflow-anchor/none''.
@@ -218,6 +217,18 @@ the implementation may wait until the anchor node is needed before computing it.
 	is |N|’s <a>scrollable overflow rectangle</a>.
 </div>
 
+<h4 id='anchor-node-invalidation'>
+Invalidation</h3>
+
+An anchor node is considered valid for a <a>scrolling box</a> until any of the following occur:
+	* The scroll position of the <a>scrolling box</a> changes (excluding adjustments originating from scroll anchoring).
+	* A <a>suppression trigger</a> suppresses a scroll adjustment.
+	* The <a>anchor node</a> is removed from the DOM.
+
+Once an anchor node has become invalid, the <a>anchor node selection algorithm</a> must be run again to compute the anchor node
+
+As a performance optimization, implementations may wait until the <a>anchor node</a> is needed before computing it.
+
 <h3 id="scroll-adjustment">
 Scroll Adjustment</h3>