add an issue on negative flexiblity, with proportinoal formula

Author: alexmog1

css3-flexbox/Overview.src.html

@@ -1704,6 +1704,29 @@ <h3 id='resolve-flexible-lengths'>
 			distribute the free space to each flexible item's main size 
 			in proportion to the item's flexibility.
 
+			<div class="issue">
+				<p>Growing and shrinking from flex basis should use different 
+				formulas for space distribution.</p>
+
+				<p>Grow proportional to <em>
+				flexibility</em>. Items with equal flexibility grow by same 
+				absolute amount.</p>
+				<pre>
+free-space = available-space - sum(flex-basis)
+main-size = flex-basis + free-space * positive-flex / sum(positive-flex)</pre>
+
+				<p>Shrink proportional to its <em>
+				flexibility and flex-basis.</em> Items with same flexibility maintain 
+				same relative size while shrinking.</p>
+				<pre>
+space-shortage = sum(flex-basis) - available-space
+shrink-factor = space-shortage / sum(flex-basis * negative-flex))
+main-size = flex-basis * (1 - shrink-factor * negative-flex))</pre>
+				<p>
+				See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16856">
+				Bug 16856</a> </p>
+			</div>
+
 		<li>
 			<strong>Fix min/max violations.</strong>
 			Clamp each item's main size by its min and max main size properties.