[css-anchor-1] Add auto-same keyword. w3c#7757

Author: tabatkins

css-anchor-1/Overview.bs

@@ -95,7 +95,8 @@ 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 | popover
-<dfn><<anchor-side>></dfn> = auto | top | left | right | bottom
+<dfn><<anchor-side>></dfn> = auto | auto-same
+           | top | left | right | bottom
 			  | start | end | self-start | self-end
 			  | <<percentage>> | center
 </pre>
@@ -131,8 +132,9 @@ The ''anchor()'' function has three arguments:
 	refers to the position of the corresponding side
 	of the [=target anchor element=].
 
-	The <dfn value for=anchor()>auto</dfn> keyword
-	indicates [=automatic anchor positioning=].
+	The <dfn value for=anchor()>auto</dfn>
+	and <dfn value for=anchor()>auto-same</dfn> keywords
+	indicate [=automatic anchor positioning=].
 	See [[#anchor-auto]] for details.
 
 	The physical <<anchor-side>> keywords
@@ -278,11 +280,16 @@ as if by an additional ''translate()'' transform.
 <h4 id=anchor-auto>
 Automatic Anchor Positioning</h4>
 
-A positioned element can use <dfn>automatic anchor positioning</dfn>
-by specifying ''anchor()/auto'' as its <<anchor-side>> value,
-causing the element to automatically stick "next to" the [=anchor element=]
-in the most obvious way,
-and flip to the other side if necessary.
+If a positioned element uses the ''anchor()'' function
+with the ''anchor()/auto'' or ''auto-same'' keywords
+in one of its [=inset properties=],
+and the opposing [=inset property=] is ''top/auto'',
+then the element is using <dfn>automatic anchor positioning</dfn>
+in that property's axis.
+This will resolve the ''anchor()'' function's <<anchor-side>>
+to the correct side of the anchor element,
+and automatically create entries in the [=position fallback list=]
+to flip the positioned element to the opposite side if necessary.
 
 [=Automatic anchor positioning=] is only active
 if the opposite [=inset property=] is ''top/auto''.
@@ -300,14 +307,18 @@ it's equivalent to ''top: anchor(bottom);'';
 when used in ''bottom: anchor(auto);'',
 it's equivalent to ''bottom: anchor(top);'';
 etc.
+The ''anchor()/auto-same'' <<anchor-side>>
+behaves as the property it's used in:
+''top: anchor(auto-same);''
+is equivalent to ''top: anchor(top);'',
+etc.
 
 Additionally,
 it automatically adds one entry
 to the beginning of the element's [=position fallback list=]
 (preceding any entries added by 'position-fallback'),
-specifying the [=inset property=] the function is used in
-as ''top/auto'',
-and the opposite property as this property's value.
+swapping the value of the [=inset property=] it's used in
+and the opposite [=inset property=].
 
 <div class=example>
 	For example, the following code using [=automatic anchor positioning=]: