[css-anchor-position-1] Move the 'use scrollable CB instead of local' to the presence of a default anchor, rather than the use of position-area or anchor-center, per WG resolution. #12552

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -573,6 +573,12 @@ is the box's <dfn>default anchor box</dfn>.
 	</pre>
 </div>
 
+If an element has a [=default anchor element=],
+then the [=scrollable containing block=] is used in place of the [=local containing block=]
+when the [=absolute-position containing block=] is generated by a [=scroll container=],
+so that the entire [=scrollable overflow area=] (typically) is available
+for positioning.
+
 ### Implicit Anchor Elements ### {#implicit}
 
 Some specifications can define that,
@@ -687,10 +693,6 @@ what area of this [=position-area grid=] to lay out the positioned box in.
 
 Values other than ''position-area/none'' have the following additional effects:
 
-* The [=scrollable containing block=] is used in place of the [=local containing block=]
-	when the [=absolute-position containing block=] is generated by a [=scroll container=],
-	so that the entire [=scrollable overflow area=] (typically) is available
-	for positioning.
 * The [=used value=] of any ''top/auto'' [=inset properties=]
 	and ''margin/auto'' [=margin properties=]
 	resolves to ''0''.
@@ -1366,10 +1368,7 @@ then it is centered (insofar as possible)
 over the [=default anchor box=]
 in the relevant axis.
 Additionally:
-* The [=scrollable containing block=] is used in place of the [=local containing block=]
-	where applicable,
-	so that the entire [=scrollable overflow area=] (typically) is available
-	for positioning.
+
 * The [=used value=] of any ''top/auto'' [=inset properties=]
 	and ''margin/auto'' [=margin properties=]
 	resolves to ''0''.