[css-overflow-5] Treat the last scroll target as active (#11320)

When a targeted scroll is performed, use that target to determine the active scroll marker. Fixes #10738.

css-overflow-5/Overview.bs

@@ -154,6 +154,28 @@ its content will be integrated into this specification,
 which will then replace it.
 Until then, this specification only contains additions and extensions to level 4.
 
+<h2 id="overflow-concepts">
+Overflow Concepts and Terminology</h2>
+
+Issue: Copy [[css-overflow-3#overflow-concepts|Level 3 content]] when final.
+
+<h3 id="scrolling">Scrolling overflow</h3>
+
+The following is added to the concepts in scrolling overflow:
+
+Every [=scroll container=] maintains a <dfn export>current scroll target</dfn>.
+It is initially <code>null</code>,
+and is reset to <code>null</code> after any scrolling operations in that [=scroll container=]
+initiated by the user,
+or any script initiated operations that do not have a target,
+or when the target is removed from the document.
+Scrolling operations in the [=scroll container=] with a target Element or PseudoElement,
+set the [=current scroll target=] to that target Element or PseudoElement.
+
+Issue: Setting the current scroll target should be defined in how to <a>scroll a target into view</a>.
+
+Issue: Use the current scroll target as an <a>anchor priority candidate</a> for scroll anchoring.
+
 <h2 id="scroll-navigation">
 Scroll navigation controls</h2>
 
@@ -304,34 +326,41 @@ Selecting The Active Scroll Marker: the '':target-current'' pseudo-class</h4>
 			1. Let <var>scroller</var> be |active|.
 			1. Let <var>targets</var> be the set of the [=scroll target=] elements whose nearest ancestor [=scroll container=] is |scroller|
 				and the [=scroll container=] elements which contain [=scroll target=] elements targeted by the [=scroll marker group=] whose nearest ancestor [=scroll container=] is |scroller|.
-			1. Let <var>primary</var> be the primary scrolling axis, assumed to be the block direction of the container's writing-mode.
-			1. Let <var>secondary</var> be the scrolling axis perpendicular to primary.
-			1. Let <var>position</var> be the 'eventual scroll position' considering ongoing scrolling operations.
-			1. For each <var>axis</var> of |primary|, followed by |secondary|:
-				1. 	Let <var>scrollport size</var> be the client size of |scroller| in the dimension |axis|.
-				1. 	For each |target| in |targets|, <a>determine the scroll-into-view position</a> of |target| in |axis|, storing this as the associated |target position| of |target|.
-				1. 	Let <var>scroll size</var> be the length of the <a>scrollable overflow area</a> of the |scroller| in the dimension |axis|.
-				1. 	Let <var>scroll range</var> be <code>|scroll size| - |scrollport size|</code>.
-				1.  If |scroll range| is greater than 0, redistribute unreachable target positions:
-					1.  Let <var>distribute range</var> be <code>min(1/8 * |scrollport size|, |scroll range| / 2)</code>.
-					1.  Let <var>before targets</var> be all |targets| whose associated |target position| is less than <code>|distribute range|</code>.
-					1.  Let <var>minimum position</var> be the minimum |target position| of |before targets|.
-					1.  Update the associated |target position| of each target in |before targets| to
-						<code>(|target position| - |minimum position|) / (|distribute range| - |minimum position|) * |distribute range|</code>.
-					1.  Let <var>after targets</var> be all |targets| whose associated |target position| is greater than <code>|scroll range| - |distribute range|</code>.
-					1.  Let <var>maximum position</var> be the maximum |target position| of |after targets|.
-					1.  Update the associated |target position| of each target in |after targets| to
-						<code>(|target position| - (|scroll range| - |distribute range|)) / (|maximum position| - (|scroll range| - |distribute range|)) * |distribute range| + (|scroll range| - |distribute range|)</code>.
-				1. 	Let |selected position| be the largest |target position|
-					where |target position| is equal to or before |position| in the |axis|,
-					or whose nearest smaller |target position| < |position| - |scrollport size| / 2 and whose |target position| < |position| + |scrollport size| / 2.
-
-				1. 	: If there is no such position,
-					::
-						Set the |selected position| to the first one.
-
-				1. Let |active| be the all of the |targets| whose associated |target position| is |selected position|.
-			1. Let |active| be the first item in |active| if it has more than one potential target.
+			1. : If |scroller| has a non-null [=current scroll target=]
+				::
+					Let active be the first item in |targets| encountered by a reverse tree order walk starting from the [=current scroll target=],
+					or the first item in tree order in |targets| if no target is found in the previous walk.
+
+				: Otherwise,
+				::
+
+					1. Let <var>primary</var> be the primary scrolling axis, assumed to be the block direction of the container's writing-mode.
+					1. Let <var>secondary</var> be the scrolling axis perpendicular to primary.
+					1. Let <var>position</var> be the 'eventual scroll position' considering ongoing scrolling operations.
+					1. For each <var>axis</var> of |primary|, followed by |secondary|:
+						1. 	Let <var>scrollport size</var> be the client size of |scroller| in the dimension |axis|.
+						1. 	For each |target| in |targets|, <a>determine the scroll-into-view position</a> of |target| in |axis|, storing this as the associated |target position| of |target|.
+						1. 	Let <var>scroll size</var> be the length of the <a>scrollable overflow area</a> of the |scroller| in the dimension |axis|.
+						1. 	Let <var>scroll range</var> be <code>|scroll size| - |scrollport size|</code>.
+						1.  If |scroll range| is greater than 0, redistribute unreachable target positions:
+							1.  Let <var>distribute range</var> be <code>min(1/8 * |scrollport size|, |scroll range| / 2)</code>.
+							1.  Let <var>before targets</var> be all |targets| whose associated |target position| is less than <code>|distribute range|</code>.
+							1.  Let <var>minimum position</var> be the minimum |target position| of |before targets|.
+							1.  Update the associated |target position| of each target in |before targets| to
+								<code>(|target position| - |minimum position|) / (|distribute range| - |minimum position|) * |distribute range|</code>.
+							1.  Let <var>after targets</var> be all |targets| whose associated |target position| is greater than <code>|scroll range| - |distribute range|</code>.
+							1.  Let <var>maximum position</var> be the maximum |target position| of |after targets|.
+							1.  Update the associated |target position| of each target in |after targets| to
+								<code>(|target position| - (|scroll range| - |distribute range|)) / (|maximum position| - (|scroll range| - |distribute range|)) * |distribute range| + (|scroll range| - |distribute range|)</code>.
+						1. 	Let |selected position| be the largest |target position|
+							where |target position| is equal to or before |position| in the |axis|,
+							or whose nearest smaller |target position| < |position| - |scrollport size| / 2 and whose |target position| < |position| + |scrollport size| / 2.
+						1. 	: If there is no such position,
+							::
+								Set the |selected position| to the first one.
+
+						1. Let |active| be the all of the |targets| whose associated |target position| is |selected position|.
+					1. Let |active| be the first item in |active| if it has more than one potential target.
 
 		1. 	Let |selected marker| be the [=scroll marker=] associated with |active|.
 			If multiple [=scroll marker=] elements are associated with |active|,