[css-overflow-5] Create scroll markers from elements. (#10617)

This changes the scroll marker primitive to be based on a regular  element within a focusgroup.

Author: flackr

css-overflow-5/Overview.bs

@@ -207,6 +207,9 @@ Overflow Controls: the 'scroll-marker-group' property and pseudo-elements
 		::scroll-marker-group { contain: size !important; }
 	
 
+	The 'scroll-marker-group' implicitly behaves as a single focusable component,
+	establishing a focusgroup.
+
 The ''::scroll-marker'' pseudo-element
 
 	When the computed 'content' value of a ::scroll-marker pseudo-element is not 'none'
@@ -215,9 +218,11 @@ Overflow Controls: the 'scroll-marker-group' property and pseudo-elements
 	on its nearest ancestor [=scroll container=].
 	These boxes are added in the [=tree order=] of their originating element.
 
-	These pseudo-elements behave as a <{button}>
-	with {{HTMLButtonElement/scrollTargetElement}} set to its originating element.
-	They can be focused and invoked.
+	These pseudo-elements have their {{HTMLLinkElement/scrollTargetElement}} set to their originating element.
+	They have a tabindex of '-1',
+	making them focusable within their '::scroll-marker-group' either by arrow key navigation within the group,
+	or via the tab key when currently active or when no other ''::scroll-marker'' is active and this is the first marker in the group,
+	ensuring the group has a guaranteed tab stop.
 
 
 Appendix A: Redirection of Overflow
@@ -885,51 +890,46 @@ Appendix B: Scroll navigation controls
 However, there is little feedback to the user regarding the current content being viewed, and
 the interaction model does not match the expectations of modern accessible UI components.
 
-This specification adds a mechanism for creating scroll navigation controls.
-The active marker, reflecting the current position, can be styled to give the user an indication of which section they are in.
-The set of markers are treated as a component following the accessibility guidelines for keyboard navigation within components.
+This specification adds a mechanism for creating scroll navigation controls within groups of <{a}> elements.
+A group is created through the focusgroup attribute
+on an ancestor containing one or more <{a}> elements whose indicated part is on the current page.
+Within each group, the active marker reflects the current scroll position, and can be styled to give the user an indication of which section they are in.
 
 Use cases include a table of contents with links to relevant contents,
 markers for scrolling carousel pages,
 and scrollable tab panels.
 
 Issue: Add images representing these examples.
 
-Issue: Explore whether scrolltarget can be more directly associated with anchor tags.
-
-<{button/scrolltarget}> attribute
-
-	The scrolltarget attribute turns a <{button}> element into a [=scroll marker control=].
-	This takes the ID of the element to target as its value.
-
-	The HTMLButtonElement.scrollTargetElement instance property
-	gets and sets the element being interacted with by the control button.
-	This is the JavaScript equivalent of the <{button/scrolltarget}> HTML attribute.
+{{HTMLLinkElement/scrollTargetElement}} attribute
 
-	A 'button' with a non-null {{HTMLButtonElement/scrollTargetElement}}
-	represents a scroll marker control that forms a scroll marker group for its nearest [=scroll container=]
-	in which exactly one control in the group can have its 'checked' state set to true.
-	A [=scroll marker control=] with a true 'checked' state can be styled by the '':checked'' pseudo-class.
+	An <{a}> element has its {{HTMLLinkElement/scrollTargetElement}}
+	initialized to the result of running the select the indicated part given the current {{Document}} and the href attribute value as the |URL|,
+	turning it into a [=scroll marker control=] if the resulting indicated part is non-null.
 
-	The [=scroll marker group=] that contains a <{button}> element a also contains all the other <{button}> elements b that fulfill the following conditions:
+	The HTMLLinkElement.scrollTargetElement instance property
+	gets and sets the element being interacted with by the anchor link and can be used to override the initial value established by the |href| property.
 
-	* The <{button}> element b has a non-null {{HTMLButtonElement/scrollTargetElement}} value.
-	* a and b's {{HTMLButtonElement/scrollTargetElement}} have the same nearest [=scroll container=].
+	An <{a}> element or ''::scroll-marker'' pseudo-element with a non-null {{HTMLLinkElement/scrollTargetElement}}
+	represents a scroll marker control which participates in a scroll marker group
+	defined by the containing ''::scroll-marker-group'' psuedo-element (for ''::scroll-marker'' pseudo-elements)
+	or nearest ancestor element establishing a focusgroup.
+	Exactly one control in a group will have its "checked" state set to true.
+	A [=scroll marker control=] with a true "checked" state can be styled by the '':checked'' pseudo-class.
 
 
-		When a [=scroll marker control=] is activated:
+		When a [=scroll marker control=] is activated by explicit invocation or arrow key focus:
 
-		1. Let element be the {{HTMLButtonElement/scrollTargetElement}} of the control.
+		1. Let element be the {{HTMLLinkElement/scrollTargetElement}} of the control.
 		1. Let block be "start".
 		1. Let inline be "start".
 		1. Scroll the element into view with behavior, block, and inline.
-		1. If activated by invocation, move focus to element.
-			If element is not focusable this will result in there being no active element,
-			but the next focus change will proceed from this element as if it were focused.
-	
+		1. 	: If the activation was triggered by invocation
+			::
+				1. Follow the hyperlink updating the URL, however retain focus on the marker element.
 
-	Issue: Moving focus to the control on activation means that the only way to control scroll markers via the keyboard is to tab into them.
-	We should retain focus after the control is activated, while still altering the point from which the next focusable element is found if the user tabs away.
+				Note: If the user tabs away the focus behavior will ensure they tab into the relevant content.
+	
 
 Scroll tracking
 
@@ -938,47 +938,45 @@ Issue: Explore whether scrolltarget can be more directly associated with anchor
 	or might directly track a user’s input
 	(e.g. touch scrolling, scrollbar dragging).
 	In either case, the user agent chooses an 'eventual scroll position' to which the scroller
-	will reach.
+	will reach. This ensures that the relevant marker is activated immediately.
 
-	This 'eventual scroll position' is used to determine the active marker.
+	This 'eventual scroll position' is used to determine the active marker within each [=scroll marker group=].
 	Since markers themselves may represent just the start of the content (e.g. headers), we consider the active marker to be the first one which we are at or beyond the scroll position of.
 
 
 		Whenever a [=scroll container=] is scrolled, or layout changes the scroll position, the user agent must run these steps to determine the active marker:
 
 		1. Let position be the 'eventual scroll position' for the scrolling operation.
-		1. Let markers be all of the [=scroll marker control=] elements which are a part of the [=scroll marker group=] for the [=scroll container=].
-		1. Let targets be the set of {{HTMLButtonElement/scrollTargetElement}} for those controls sorted in [=tree order=].
-		1. Let selected be the first element in targets, or null if targets is empty.
-		1. For each target in targets:
-			1. Set selected to target.
-			1. Let targetPosition be the position that would be scrolled to if we scroll target into view.
-			1.	: If targetPosition's scroll block offset is less than or equal to position's scroll block offset, and
-					targetPosition's scroll inline offset is less than or equal to position's scroll inline offset.
-				::
-					Update selected to target.
-					Break.
-		1. 	: If selected is not null,
-			::
-				1. Let marker be the first control in markers whose {{HTMLButtonElement/scrollTargetElement}} is selected.
-				1. Set the 'checked' state of marker to true.
-			: Otherwise,
-			::
-				Set the 'checked' state of all controls in markers to false.
-	
+		1. For each focusgroup group containing one or more [=scroll marker control=] elements whose {{HTMLLinkElement/scrollTargetElement}} is a descendant of [=scroll container=]:
 
-	Issue: Should we allow for none of the markers to be currently active, e.g. if not yet scrolled past the position of the first marker.
+			1. 	Let markers be all of the [=scroll marker control=] elements which are a part of the [=scroll marker group=] for the [=scroll container=].
+			1. 	Let targets be the {{HTMLLinkElement/scrollTargetElement}}s of |markers|, associated with the item of |markers| they came from, and sorted in [=tree order=].
+			1. 	For each |target| in |targets|, determine the scroll-into-view position of |target|, storing this as the associated |target position| of |target|.
+			1. 	Let |selected target| be the largest-indexed item of |targets|
+				whose associated |target position| is equal to or before |position| in both the block and inline axises in the current writing mode direction of the [=scroll container=].
 
-Focus behavior
+				Issue: When the next marker is closer to being aligned than the previous we should use the next marker, in a manner similar to mandatory snap point selection.
 
-	A [=scroll marker control=] is only focusable if it is 'checked'. Within a group, exactly one marker is 'checked' at a time.
+			1. 	: If there is no such item,
+				::
+					Set the "checked" state of all |markers| in the |group| to false and return.
+
+			1. 	Let |selected marker| be the marker associated with |selected target|.
+				If multiple items of |markers| are associated with |selected target|,
+				set |selected marker| to be the marker that is earliest in tree order.
+			1. 	Set the "checked" state of |selected marker| to true.
+			1.	: If the active element was the last-focused element of the |group|,
+					::
+						Focus |selected marker|
+			1. 	Set the last-focused element of the |group| to |selected marker|.
+			1. 	Set the "checked" state of all other |markers| in the |group| to false.
+	
 
-	When such a control is focused,
-	* The down arrow or right arrow move focus to and activate the next control from its [=scroll marker group=].
-	* The up arrow or left arrow move focus to and activate the previous control from its [=scroll marker group=].
-	* Space or Enter invoke the control.
+Focus behavior
 
-	Issue: We should be able to tab away from the target immediately after using arrow navigations rather than requiring activating the control first.
+	When a [=scroll marker control=] is activated,
+	the next tabindex-ordered focus navigation will focus the {{HTMLLinkElement/scrollTargetElement}} if it is focusable,
+	otherwise, it will find the next focusable element from this element as though it were focused.
 
 
 Appendix C: Privacy Considerations