[css-anchor-position-1] Rather than adjusting double-auto insets to achieve centering, just zero them and adjust available space instead.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -780,20 +780,14 @@ makes this case extremely simple:
 if the positioned element has a [=default anchor element=],
 then it is aligned so as to center itself
 over the [=default anchor element=]
-in the appropriate axis.
+in the relevant axis.
 
 Additionally,
-if both [=inset properties=] in the appropriate axis are ''top/auto'',
-they resolve to the offsets necessary
-to create the largest rectangle possible
-that is centered over the [=default anchor element=]
-and does not overflow the [=containing block=].
-If only one [=inset property=] is ''top/auto'',
-it resolves to ''0''.
-
-Issue: See <a href="https://github.com/w3c/csswg-drafts/issues/9124">Issue 9124</a>
-about making this the behavior in general
-for non-''justify-self/auto'' alignment values.
+any ''top/auto'' [=inset properties=] resolve to ''0''.
+However, the [=available space=] for the positioned element in the relevant axis
+is reduced to the size of the largest rectangle
+that is centered on the [=default anchor element=]
+and doesn't overflow the [=inset-modified containing block=].
 
 If the element is not [=absolutely positioned=],
 or does not have a [=default anchor element=],