[css-anchor-position-1] Clarify timing of anchors-visible checks. #12732

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -2226,11 +2226,11 @@ depending on some commonly needed layout conditions.
 	'overflow', or other effects (such as [=paint containment=])
 	that clip to the [=overflow clip edge=].
 
-	ISSUE(12732): Define timing of visibility checks.
-	<!--
 	Whether or not |anchor| is [=clipped by intervening boxes=]
-	is determined immediately prior to triggering {{ResizeObserver}}s.
-	-->
+	must be checked after [=updating content relevancy for a document=]
+	(see 'content-visibility' in [[css-contain-2]])
+	and before running any {{IntersectionObserver}},
+	and may also be checked at other times to improve responsiveness.
 
 	Note: This means that if an abspos is next to its anchor in the DOM,
 	for example,
@@ -2633,7 +2633,8 @@ Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12952">Issue 12952</a>)
 * Clarify that ''flip-block'', ''flip-inline'', and ''flip-start'' use the [=writing mode=] of the [=containing block=].
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12869">Issue 12869</a>)
-* Add 'clip-path' to the list of clipping effects considered for ''anchors-visible''.
+* Add 'clip-path' to the list of clipping effects considered for ''anchors-visible''
+	and clarify the timing of its checks.
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12732">Issue 12732</a>)
 * Fix error where base styles were accidentally left out of the [=position options list=].
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12890">Issue 12890</a>)