[css-anchor-position-1] Make anchor-center also use scrollable containing block #12952

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -648,7 +648,7 @@ 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=]
-	where applicable,
+	when the [=absolute-position containing block=] is generated by a [=scroll container=],
 	so that the entire [=scrollable overflow area=] (typically) is available
 	for positioning.
 * Any ''top/auto'' [=inset properties=] resolve to ''0''.
@@ -1312,9 +1312,12 @@ if the positioned box has a [=default anchor box=],
 then it is centered (insofar as possible)
 over the [=default anchor box=]
 in the relevant axis.
-
-Additionally,
-any ''top/auto'' [=inset properties=] resolve to ''0''.
+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.
+* Any ''top/auto'' [=inset properties=] resolve to ''0''.
 
 If the box is not [=absolutely positioned=],
 or does not have a [=default anchor box=],
@@ -2605,38 +2608,10 @@ No Privacy issues have been raised against this document.
 Changes {#changes}
 =======
 
-Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20250509/">9 May 2025 Working Draft</a>:
-
-* Rename <css>x/y-self-start/end</css> keywords to <css>self-x/y-start/end</css> for 'position-area'
-	due to persistent confusion about the ordering.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/12749">Issue 12749</a>)
-* Make 'position-area' use the (newly defined) [=scrollable containing block=]
-	when the box's [=containing block=] is a [=scroll container=].
-	(<a href="https://github.com/w3c/csswg-drafts/issues/10861">Issue 10861</a>)
-* Make default 'position-area' alignment for single-''inset/auto'' 'inset' axes align to the non-''inset/auto'' side.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/12512">Issue 12512</a>)
-* Make transforms affect the [=anchor box=] geometry.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/8584">Issue 8584</a>)
-* Add the ''place-self/dialog'' value to 'align-self' and 'justify-self'
-	to solve regrettable default UA stylesheet rules for HTML popovers.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/10258">Issue 10258</a>)
-* Clarify how fragmentation affects the [=anchor box=] geometry.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/12287">Issue 12287</a>)
-* Clarify whe the [=last successful position option=] is cleared.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/12577">Issue 12577</a>)
-* Clarify how tree-scoped names anchor are matched for 'anchor-scope' vs 'position-anchor'.
-	(Issues <a href="https://github.com/w3c/csswg-drafts/issues/7916">7916</a>, <a href="https://github.com/w3c/csswg-drafts/issues/9408">9408</a>, and <a href="https://github.com/w3c/csswg-drafts/issues/5984">5984</a>)
-* Clarify how top layer interacts with finding the [=acceptable anchor element=]
-	and rewrite the rules for valid anchors to be easier to understand.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/11602">Issue 11602</a>)
-* Fix grammar of 'position-try-fallbacks' to exclude lists of ''position-area/none'' keywords.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/12838">Issue 12838</a>)
-* Various small fixes and clarifications.
-	(<a href="https://github.com/w3c/csswg-drafts/issues/11028">Issue 11028</a>,
-	 <a href="https://github.com/w3c/csswg-drafts/issues/11080">Issue 11080</a>,
-	 <a href="https://github.com/w3c/csswg-drafts/issues/12058">Issue 12058</a>,
-	 <a href="https://github.com/w3c/csswg-drafts/issues/12636">Issue 12636</a>,
-	 <a href="https://github.com/w3c/csswg-drafts/issues/12653">Issue 12653</a>,
-	 <a href="https://github.com/w3c/csswg-drafts/issues/12751">Issue 12732</a>)
-
-See also <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20250509/#changes">Previous Changes</a>.
+Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20251007/">7 October 2025 Working Draft</a>:
+
+* Define ''anchor-center'' to also use the [=scrollable containing block=]
+	so that it doesn't trigger overflow alignment when positioned outside the [=local containing block=].
+	(<a href="https://github.com/w3c/csswg-drafts/issues/12952">Issue 12952</a>)
+
+See also <a href="https://www.w3.org/TR/2025/WD-css-anchor-position-1-20251007/#changes">Previous Changes</a>.