[css-containment] Fix layout containment and fragmentainer interaction

Author: frivoal

css-containment/Overview.bs

@@ -138,6 +138,37 @@ Layout Containment</h3>
 
 	2. The element must be a <a>formatting context</a>.
 
+	3. If a <a>fragmentation context</a> participates in <a>layout containment</a>,
+		the first element with <a>layout containment</a>
+		affecting the <a>fragmentation context</a>
+		must “trap” the remainder of the <a>fragmented flow</a>.
+		<a>Fragmentation</a> must not continue past the <a>layout containment</a> boundary,
+		and the last <a>fragmentation container</a>
+		within the first <a>layout containment</a> boundary
+		is treated as if it is the last <a>fragmentation container</a>
+		in its <a>fragmentation context</a>.
+
+		If subsequent <a>fragmentation containers</a> in the <a>fragmentation context</a>
+		are only generated when more content remains in the <a>fragmented flow</a>,
+		then they are not generated.
+		If they would exist regardless,
+		they remain part of the <a>fragmentation context</a>,
+		but do no receive any content from the <a>fragmented flow</a>.
+
+		Specifically:
+			- <a spec=css-regions>CSS Regions</a> following the one which traps the content
+				are still considered part of the <a spec=css-regions>region chain</a>
+				as returned by the {{getRegions()}} method
+				of the {{NamedFlow}} interface.
+			- the {{regionOverset}} attribute of the {{Region}} interface
+				of the region which traps the content
+				is set to ''overset'' if the content doesn't fit,
+				even if it is not the last region in the region chain.
+			- If the computed value of the 'continue' property
+				on an element with <a>layout containment</a>
+				would otherwise have been ''continue/auto'' or ''continue/fragments'',
+				it must instead compute to ''continue/overflow''.
+
 	Possible optimizations that can be enabled by <a>layout containment</a> include (but are not limited to):
 
 	1. When the style or contents of a descendant of the containing element is changed,