[css-scroll-anchoring-1] Add priority candidates for scroll anchor selection #5018

css-scroll-anchoring-1/Overview.bs

@@ -29,6 +29,13 @@ spec:css22;
 		text:width
 	type:dfn; text:line box
 </pre>
+<pre class="anchors">
+spec: html; urlPrefix: https://html.spec.whatwg.org/multipage/;
+    urlPrefix: interaction.html
+        type: dfn;
+            text:DOM anchor
+            text:focused area of the document
+</pre>
 
 <h2 id=intro>
 Introduction</h2>
@@ -76,7 +83,8 @@ Anchor Node Selection</h3>
 
 Each <a>scrolling box</a> aims to select an <a>anchor node</a>
 that is deep in the DOM
-and close to the block start edge of its <a>optimal viewing region</a>.
+and either should be prioritized as an important DOM node or is close to the
+block start edge of its <a>optimal viewing region</a>.
 
 Note: If the user agent does not support the 'scroll-padding' property,
 the optimal viewing region of the scrolling box is equivalent to its <a>content area</a>.
@@ -85,16 +93,40 @@ An anchor node can be any <a>box</a> except one for a non-<a>atomic inline</a>.
 The anchor node is always a <a>descendant</a> of the <a>scrolling box</a>.
 In some cases, a scrolling box may not select any anchor node.
 
+An element |C| is a <dfn id="anchor-viable-candidate">viable candidate</dfn>
+for becoming a scroll anchor for a scrolling box |S| if it meets all of the
+following criteria:
+  * |C| is an element that is not a non-<a>atomic inline</a>.
+  * |C| is <a>partially visible</a> or <a>fully visible</a> in |S|
+  * |C| is a descendant of |S|
+  * |C| is not in an <a>excluded subtree</a>
+  * None of the ancestors of |C| up to |S| are in an <a>excluded subtree</a>
+
+Some elements are considered to be <dfn
+id="anchor-priority-candidates">priority candidates</dfn> for anchor selection:
+  1. The [=DOM anchor=] of the [=focused area of the document=].
+  2. An element containing the current active selected match of the
+      find-in-page user-agent algorithm. If the match spans multiple elements, then
+      consider only the first such element.
+
+Note that if the <a>priority candidate</a> is a non-<a>atomic inline</a>
+element, then instead consider its nearest ancestor element that is not a
+non-atomic inline element as the priority candidate.
+
 <div algorithm>
 	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
 		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 DOM child |N| of the element or document associated with |S|,
+	2. Otherwise, for each <a>priority candidate</a> |PC| in order specified,
+		check if |PC| is a <a>viable candidate</a> in |S|. If so, select it as an
+		anchor node and terminate.
+	3. 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.
+
 </div>
 
 <div algorithm>