Added some text about stretchable margins.

Author: bert-github

css3-box/Overview.src.html

@@ -1505,11 +1505,34 @@ <h3>The margin properties</h3>
 <div class=issue>
 <p>Add a stretch value to allow vertical distribution of blocks in a
 fixed-height containing block? css3-flexbox currently (May 2012)
-proposes to use ''auto'', but in normal flow we need something else,
-because ''auto'' already means ''0'' there.
+proposes to use ''auto'', but in normal flow we probably need
+something else, because ''auto'' already means ''0'' there.
 
-<pre class=prod><var>&lt;stretch&gt;</var> = fill( [ <var>&lt;length&gt;</var>  | <var>&lt;percentage&gt;</var> ] )</pre>
+<pre class=prod><dfn>&lt;stretch&gt;</dfn> = fill( [ <var>&lt;length&gt;</var>  | <var>&lt;percentage&gt;</var> ] )</pre>
+
+<p>This sets an elastic margin with a minimum thickness. Maybe it is
+enough without the minimum thickness, i.e.:
+
+<pre class=prod><var>&lt;stretch&gt;</var> = fill</pre>
+
+<p>Collapsing margins of which at least one has a
+<var>&lt;stretch&gt;</var> value would yield 'fill(<var>M</var> -
+<var>N</var>), or maybe just 'fill'.
+
+<p>To find the size of such a collapsed margin, find the nearest
+enclosing containing block in the same flow that has a fixed height,
+call that height <var>H1</var>. Compute the height of the content in
+that containing block (while treating 'fill(<var>P</var>)' as if it
+was simply <var>P</var>) and call that height <var>H2</var>. Count the
+number <var>N</var> of stretchable margins. If <var>H2</var> &lt;
+<var>H1</var>, then increase all the stretchable margins by
+(<var>H1</var> - <var>H2</var>)/<var>N</var>. If there is no such
+enclosing block, 'fill(<var>P</var>)' simply means <var>P</var>.
+
+<p>In a multi-column element, each column box is a separate containing
+block for the purpose of stretching the margins.
 </div>
+
 <p>These properties set the thickness of the <em>margin area</em>. The value
 may be negative.