[css-conditional-5] Size query containers and box tree re-ordering #11213 (#12960)

* [css-conditional-5] Size query containers and box tree re-ordering #11213

Per resolution:

#11213 (comment)

* Apply suggestion from @tabatkins

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Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: lilles

css-conditional-5/Overview.bs

@@ -437,6 +437,22 @@ Container Queries</h2>
 	can be conditioned by querying against it,
 	using the ''@container'' [=conditional group rule=].
 
+	An ancestor element that generates a box is not an eligible container for
+	[=container size queries=] if that box is not an ancestor box of any boxes
+	generated by the querying element.
+
+	<div class=note>
+		There are cases where the box for a [=pseudo-element=] is generated as a
+		sibling box of its originating element's box. If we allowed querying the size
+		of a sibling box, it would introduce layout cycles.
+		
+		For example, the ''::scroll-marker-group'' and ''::scroll-button()'' pseudo-elements
+		generate sibling boxes of their originating element's box.
+		These pseudo-elements will not be able to query their originating scroller
+		for [=container size queries=]. They will, however, be able to query other
+		eligible [=query container=] ancestors.
+	</div>
+
 	<div class=example>
 		For example, we can define the main content area and sidebar as containers,
 		and then describe a ''.media-object'' that changes