[css-anchor-position-1] Adopt Xiaocheng's suggestion for fixing the fallback check. w3c#8059.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -24,18 +24,18 @@ spec:css-cascade-5; type:dfn; text:property
 <style>
 /* Put nice boxes around each algorithm. */
 [data-algorithm]:not(.heading) {
-  padding: .5em;
-  border: thin solid #ddd; border-radius: .5em;
-  margin: .5em calc(-0.5em - 1px);
+	padding: .5em;
+	border: thin solid #ddd; border-radius: .5em;
+	margin: .5em calc(-0.5em - 1px);
 }
 [data-algorithm]:not(.heading) > :first-child {
-  margin-top: 0;
+	margin-top: 0;
 }
 [data-algorithm]:not(.heading) > :last-child {
-  margin-bottom: 0;
+	margin-bottom: 0;
 }
 [data-algorithm] [data-algorithm] {
-    margin: 1em 0;
+	margin: 1em 0;
 }
 </style>
 
@@ -91,12 +91,12 @@ to refer to the position of one or more [=anchor elements=].
 The ''anchor()'' function resolves to a <<length>>.
 
 <pre class=prod>
-&lt;anchor()> = anchor( <<anchor-element>>? <<anchor-side>>, <<length-percentage>>? )
-<dfn><<anchor-element>></dfn> = <<dashed-ident>> | implicit
-<dfn><<anchor-side>></dfn> = auto | auto-same
-           | top | left | right | bottom
-			  | start | end | self-start | self-end
-			  | <<percentage>> | center
+	&lt;anchor()> = anchor( <<anchor-element>>? <<anchor-side>>, <<length-percentage>>? )
+	<dfn><<anchor-element>></dfn> = <<dashed-ident>> | implicit
+	<dfn><<anchor-side>></dfn> = auto | auto-same
+	           | top | left | right | bottom
+	           | start | end | self-start | self-end
+	           | <<percentage>> | center
 </pre>
 
 The ''anchor()'' function has three arguments:
@@ -899,21 +899,44 @@ Probably should introduce a <css>smooth</css> keyword
 to 'position-fallback'
 to trigger automatic "animation" of the fallback'd properties.
 
-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=].
-
-Note: Descendants overflowing the anchored block
-don't affect this.
-
-The properties of the first non-overflowing entry
-(or the last attempted entry, if none succeeded),
+<div algorithm>
+	To <dfn>determine the position fallback styles</dfn> of an element |el|:
+
+	1. Let |base styles| be the current used styles of |el|.
+
+	2. [=list/For each=] |fallback styles| in the [=position fallback list=]:
+
+		1. Apply the styles in |fallback styles| to |el|,
+			overriding the corresponding properties in |base styles|.
+
+			Perform any specified/computed/used-value time normalizations
+			that are required to make the overridden styles into [=used values=]
+			(such as resolving [=math functions=], etc).
+
+			Let |adjusted styles|
+			be |el|'s styles after these adjustments.
+
+		2. If |el| has a [=snapshotted scroll offset=],
+			then shift |el|'s margin box's position by the offsets.
+
+			Also, if any of |el|'s [=inset properties=] are non-auto,
+			add the [=snapshotted scroll offset=] for the appropriate axis
+			to their values.
+			Recalculate |el|'s [=inset-modified containing block=]
+			using these shifted values
+			to obtain the |scroll-adjusted IMCB|.
+
+		3. If |el|'s margin box is fully contained within the |scroll-adjusted IMCB|,
+			or |fallback styles| is the final entry in the [=position fallback list=],
+			then use |adjusted styles| for |el|
+			and exit this algorithm.
+
+	Note: Descendants overflowing |el|
+	don't affect this calculation,
+	only |el|'s own [=margin box=].
+</div>
+
+The styles returned by [=determining the position fallback styles=]
 are taken as the final values for the specified properties.
 
 Implementations may choose to impose an implementation-defined limit