[css-multicol-1] define spanning element as out-of-flow. Fixes #1072

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -939,7 +939,8 @@ Spanning Columns</h2>
 		</dd>
 		<dt><dfn>all</dfn></dt>
 		<dd>
-			The element spans across all columns of the nearest multicol ancestor 
+			The element is taken [=out-of-flow=] 
+			and spans across all columns of the nearest multicol ancestor 
 			in the same [=block formatting context=].
 			Content in the normal flow that appears before the element 
 			is automatically balanced across all columns 
@@ -972,6 +973,9 @@ Spanning Columns</h2>
 	An element that spans more than one column is called a <dfn export>spanning element</dfn> 
 	and the box it creates is called a <dfn export>spanner</dfn>.
 
+	A spanning element is taken [=out-of-flow=], leaving a forced break. 
+	This does not affect the <a href="https://www.w3.org/TR/CSS2/zindex.html">painting order</a> [[!CSS21]] of the spanning element. 
+
 	<div class="example">
 		In this example, an <code>h2</code> element has been added to the sample document after the sixth sentence 
 		(i.e., after the words "the leg of a"). 
@@ -1435,6 +1439,7 @@ This appendix is <em>informative</em>.
 
 <h3 id="changes-from-20180528">Changes from the <a href="https://www.w3.org/TR/2018/WD-css-multicol-1-20180528/">Working Draft (WD) of 28 May 2018</a></h3>
 <ul>
+	<li>Clarified the spec to explain that a spanning element is taken out-of-flow, leaving a forced break. Added the paragraph, "A spanning element is taken out-of-flow, leaving a forced break. This does not affect the painting order of the spanning element." <a href="https://github.com/w3c/csswg-drafts/issues/1072#issuecomment-468087733">Resolved 28 Feb 2019</a>.</li>
 	<li>Moved the definition of the 'column-gap' property to [[CSS3-ALIGN]] and added a paragraph detailing the specifics of 'column-gap' in multicol:
 	<br />"In a multi-column formatting context the used value of normal for the column-gap property is 1em. This ensures columns are readable when the initial values are used. If there is a column rule between columns, it will appear in the middle of the gap." <a href="https://github.com/w3c/csswg-drafts/issues/3641">Resolved 4 June 2019.</a></li>
 	<li>Reworded the section <a href="#the-multi-column-model">The multi-column model</a>, based on input from Morten Stenshorne in <a href="https://github.com/w3c/csswg-drafts/issues/2203#issuecomment-431695940">issue 2203</a>.</li>