[css-contain] layout containment causes stacking contexts

Closes w3c#2942

Author: frivoal

css-contain-1/Overview.bs

@@ -304,7 +304,9 @@ Layout Containment</h3>
 
 	4. The element acts as a containing block for absolutely positioned and fixed positioned descendants.
 
-	5. [=Forced breaks=] are allowed within elements with [=layout containment=],
+	5. The element creates a <a>stacking context</a>.
+
+	6. [=Forced breaks=] are allowed within elements with [=layout containment=],
 		but do not propagate to the parent as otherwise described in [[CSS-BREAK-3#break-between]].
 
 		Note: This introduces the previously non-existent possibility that [=forced breaks=]
@@ -514,6 +516,7 @@ This appendix is <em>informative</em>.
 <h3 id="2018-05-24-changes">Changes from the
 <a href="https://www.w3.org/TR/2018/CR-css-contain-1-20180524/">Candidate Recommendation of 24 May 2018</a></h3>
 
+* Layout containment causes the element to establish a new stacking context
 * Size containment does not apply to tables
 * Clarify that the columns and grid properties affect the size of size-contained elements
 * Change the animation type of the contain property from discrete to not animatable

css-contain-1/issues-2018-cr.html

@@ -81,6 +81,16 @@ <h1>CSS Contain Level 1 Disposition of Comments for 2018-05-24 CR</h1>
 Comment:  <a href='https://github.com/w3c/csswg-drafts/issues/2800'>https://github.com/w3c/csswg-drafts/issues/2800</a>
 Closed:   Accepted
 Resolved: Editorial</pre>
+<pre class=' open' id='issue-7'>
+Issue 7. <a href='#issue-7'>#</a>
+Summary:  contain:layout is a CB for fixpos/abspos descendants but doesn't establish a stacking context
+From:     David Baron
+Comment:  <a href='https://github.com/w3c/csswg-drafts/issues/2942'>https://github.com/w3c/csswg-drafts/issues/2942</a>
+Response: <a href='https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406716817'>https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406716817</a>
+Response: <a href='https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406735520'>https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406735520</a>
+Response: <a href='https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-407956323'>https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-407956323</a>
+Response: <a href='https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-408244344'>https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-408244344</a>
+Open</pre>
 <script>
 (function () {
     var sheet = document.styleSheets[0];

css-contain-1/issues-2018-cr.txt

@@ -51,3 +51,13 @@ From:     Daniel Holbert
 Comment:  https://github.com/w3c/csswg-drafts/issues/2800
 Closed:   Accepted
 Resolved: Editorial
+----
+Issue 7.
+Summary:  contain:layout is a CB for fixpos/abspos descendants but doesn't establish a stacking context
+From:     David Baron
+Comment:  https://github.com/w3c/csswg-drafts/issues/2942
+Response: https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406716817
+Response: https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-406735520
+Response: https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-407956323
+Response: https://github.com/w3c/csswg-drafts/issues/2942#issuecomment-408244344
+Open