[css-anchor-1] Whoops, forgot that position fallback immediately overrides base styles, rather than using base styles as the first entry. w3c#7757

Author: tabatkins

css-anchor-1/Overview.bs

@@ -314,11 +314,19 @@ 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'),
-swapping the value of the [=inset property=] it's used in
-and the opposite [=inset property=].
+if the element has ''position-fallback: none'',
+[=automatic anchor positioning=] causes the element
+to gain a [=position fallback list=]
+consisting of two entries:
+
+* one containing all the base-style properties on the element
+	that are valid to use in ''@try'' rules,
+	with ''anchor()/auto''/''auto-same'' keywords
+	resolved to their appropriate side.
+* one containing the same,
+	but with the [=inset properties=] in each axis swapped,
+	and the ''anchor()/auto''/''auto-same'' keywords
+	resolved to the opposite sides as well.
 
 <div class=example>
 	For example, the following code using [=automatic anchor positioning=]:
@@ -335,10 +343,13 @@ and the opposite [=inset property=].
 	<pre highlight=css>
 	.foo {
 		position: absolute;
-		top: calc(.5em + anchor(--foo bottom));
 		position-fallback: --flip;
 	}
 	@position-fallback --flip {
+		@try {
+			top: calc(.5em + anchor(--foo bottom));
+			bottom: auto;
+		}
 		@try {
 			top: auto;
 			bottom: calc(.5em + anchor(--foo top));