[css-anchor-1] Second attempt at fixing 'acceptable anchor element' rules #12830

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -425,24 +425,26 @@ in anchor positioning.
 
 * |possible anchor| is laid out strictly before |positioned el|,
 aka one of the following is true:
-* Both |positioned el| and |possible anchor| are [=absolutely positioned=],
-and |positioned el|'s [=containing block=] is
-an ancestor of |possible anchor|'s [=absolute positioning containing block=]
-in the [=containing block chain=].
-
-Note: The viewport is the highest possible containing block,
-and an ancestor of all other containing blocks.
-The [=initial containing block=] is a child of the viewport,
-and an ancestor of all [=containing blocks=]
-generated by elements not in a [=top layer=].
-* Both |positioned el| and |possible anchor| are [=absolutely positioned=]
-and have the same [=containing block=],
-and either |positioned el| is [=in a higher top layer=] than |possible anchor|,
-or they're both in the same [=top layer=]
-and |positioned el| is later in [=flat tree=] order than |possible anchor|.
-* |possible anchor| is not [=absolutely positioned=],
-and its [=containing block=]
-is either the same as or a descendant of |positioned el|'s [=containing block=].
+* |possible anchor| and |positioned el| share a [=containing block=]
+and either
+* |possible anchor| is [=in a lower top layer=] than |positioned el|, or
+* they both exist in the same [=top layer=],
+ but |possible anchor| is either not [=absolutely positioned=]
+ or occurs earlier in the [=flat tree=] order than |positioned el|
+* The element generating |possible anchor|'s [=containing block=]
+(if one exists)
+is an [=acceptable anchor element=] for |positioned el|
+
+Issue: “Share a containing block” isn't strictly correct;
+for example, two [=absolutely positioned boxes=]
+contained by the same [=grid container=] but with different 'grid-area' values
+get different [=containing blocks=],
+but should treat them as equivalent for this case.
+On the other hand, a scroller's [=fixed containing block=]
+and that same scroller's [=local containing block=]
+are using different [=containing blocks=] for this purpose
+(for example the [=initial fixed containing block=] contains the [=initial containing block=]).
+Need a better notion of “equivalent” [=containing blocks=] here.
 
 * If |possible anchor| is in the [=skipped contents=] of another element,
 then |positioned el| is in the [=skipped contents=] of that same element.