Added paragraph about the effect of 'visiblity:collapse' on flexbox items.

Author: tabatkins

css3-flexbox/Overview.html

@@ -140,6 +140,10 @@ <h2 class="no-num no-toc" id=table>Table of contents</h2>
 
      <li><a href="#flex-items"><span class=secno>2.2. </span> Flexbox
       Items</a>
+      <ul class=toc>
+       <li><a href="#abspos-flexbox-items"><span class=secno>2.2.1. </span>
+        Positioning Absolutely Positioned Flexbox Items</a>
+      </ul>
     </ul>
 
    <li><a href="#ordering-orientation"><span class=secno>3. </span> Ordering
@@ -466,6 +470,16 @@ <h3 id=flex-items><span class=secno>2.2. </span> Flexbox Items</h3>
     positioned and thus out of flow.</p>
   </div>
 
+  <p>Similar to table cells, flexbox items respond specially to the
+   &lsquo;<code class=css>collapse</code>&rsquo; value of the &lsquo;<code
+   class=property>visibility</code>&rsquo; property. When a flexbox item is
+   set to &lsquo;<code class=css>visibility: collapse;</code>&rsquo;, the
+   item does not generate a box at all (identical to setting &lsquo;<code
+   class=css>display: none;</code>&rsquo;).
+
+  <h4 id=abspos-flexbox-items><span class=secno>2.2.1. </span> Positioning
+   Absolutely Positioned Flexbox Items</h4>
+
   <p>Absolutely positioned children of a flexbox are not <a
    href="#flexbox-item"><i>flexbox items</i></a>, but their "static position"
    (their position when the &lsquo;<code
@@ -1667,7 +1681,7 @@ <h2 class=no-num id=index>Index</h2>
     title=flex-pack><strong>5.1.</strong></a>
 
    <li>hypothetical neighbors, <a href="#hypothetical-neighbors"
-    title="hypothetical neighbors"><strong>2.2.</strong></a>
+    title="hypothetical neighbors"><strong>2.2.1.</strong></a>
 
    <li>justify, <a href="#flex-line-pack-justify"
     title=justify><strong>6.1.</strong></a>, <a href="#flex-pack-justify"

css3-flexbox/Overview.src.html

@@ -187,6 +187,8 @@ <h3 id='flex-items'>
 		<p>Notice that block element "not-an-item6.3" is not a separate flexbox item, because it is contained inside an inline element which is being wrapped into an anonymous flex item.  Similarly, the block element "not-an-item1.5" is not a flexbox item, because it's absolutely positioned and thus out of flow.</p>
 	</div>
 
+	<p>Similar to table cells, flexbox items respond specially to the ''collapse'' value of the 'visibility' property.  When a flexbox item is set to ''visibility: collapse;'', the item does not generate a box at all (identical to setting ''display: none;'').</p>
+
 <h4 id='abspos-flexbox-items'>
 Positioning Absolutely Positioned Flexbox Items</h4>