[css-anchor-1] Revive 'anchor-scroll', as it does appear necessary to allow proper use of the compositing thread. tabatkins/specs#91

Author: tabatkins

css-anchor-1/Overview.bs

@@ -143,7 +143,12 @@ resolves to the <<length>>
 that would align the edge
 of the positioned elements' [=inset-modified containing block=]
 corresponding to the property the function appears in
-with the specified border edge of the [=target anchor element=].
+with the specified border edge of the [=target anchor element=],
+assuming that all [=scroll containers=]
+between the [=target anchor element=]
+and the positioned element's [=containing block=]
+are scrolled to their initial scroll position
+(but see 'anchor-scroll').
 
 If the [=target anchor element=] is [=fragmented=],
 the axis-aligned bounding rectangle
@@ -152,12 +157,11 @@ of the fragments' border boxes is used instead.
 Issue: Do we need to control which box we're referring to,
 so you can align to padding or content edge?
 
-Issue: Per flackr/iank feedback,
-this should be able to respond to the scroll position
-of the anchor element,
-both for positioning and for fallback,
-in a way essentially identical to @scroll-timeline.
-Precise timing + details to be filled in.
+If the positioned element
+has a [=snapshotted scroll offset=],
+then it is additionally visually shifted
+by those offsets,
+as if by an additional ''translate()'' transform.
 
 <div class=example>
 	For example,
@@ -267,6 +271,83 @@ whichever is in the specified axis)
 of the [=target anchor element=].
 
 
+<!-- Big Text: a-scroll -->
+
+Taking Scroll Into Account: the 'anchor-scroll' property {#scroll}
+------------------------------------------------------------------
+
+<pre class=propdef>
+Name: anchor-scroll
+Value: none | <<dashed-ident>>
+Initial: none
+Inherited: no
+Applies to: [=absolutely-positioned=] elements
+Animation Type: discrete
+</pre>
+
+Because scrolling is often done in a separate thread from layout in implementations for performance reasons,
+but ''anchor()'' can result in both positioning changes
+(which can be handled in the scrolling thread)
+and layout changes
+(which cannot),
+''anchor()'' is defined to assume
+all the [=scroll containers=] between the anchor element
+and the positioned element's containing block
+are at their initial scroll position.
+This means a positioned element
+will <em>not</em> be aligned with its anchor
+if any of the scrollers are <em>not</em> at their initial positions.
+
+The 'anchor-scroll' property allows an author to compensate for this,
+without losing the performance benefits of the separate scrolling thread,
+so long as the positioned element
+is only anchoring to a single anchor element.
+Its values are:
+
+<dl dfn-type=value dfn-for=anchor-scroll>
+	: <dfn>none</dfn>
+	:: No effect.
+
+	: <dfn><<dashed-ident>></dfn>
+	::
+		The scroll offsets of the [=target anchor element=]
+		with the name given by the <<dashed-ident>>
+		will be compensated for in positioning and fallback.
+
+	: <dfn>implicit</dfn>
+	::
+		The scroll offsets of the [=implicit anchor element=]
+		will be compensated for in positioning and fallback.
+</dl>
+
+<div algorithm="compensate for scroll">
+	If 'anchor-scroll' is not ''anchor-scroll/none''
+	on an [=absolutely-positioned=] element |query el|,
+	and there is a [=target anchor element=] for |query el|
+	with either the name given by 'anchor-scroll'
+	(if its value is a <<dashed-ident>>)
+	or no name
+	(if its value is ''implicit''),
+	and at least one ''anchor()'' function on |query el|
+	refers to the same [=target anchor element=],
+	then |query el| has a <dfn>snapshotted scroll offset</dfn>,
+	which is a pair of lengths
+	representing a vertical and horizontal offset.
+
+	The [=snapshotted scroll offset=]
+	is the sum of the offsets from the [=initial scroll position=]
+	of all [=scroll container=] ancestors of the [=target anchor element=],
+	up to but not including |query el|'s [=containing block=].
+
+	Issue: Define the precise timing of the snapshot:
+	updated each frame,
+	before style recalc.
+</div>
+
+
+
+
+
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@@ -579,6 +660,9 @@ To determine which entry is selected,
 [=list/iterate=] over the [=position fallback list=],
 applying the properties of each entry in turn
 according to the standard cascade rules,
+and additionally shifting the element's [=margin box=]
+according to its [=snapshotted scroll offset=]
+(if it has one),
 and determining whether or not the element's [=margin box=]
 overflows its [=containing block=].