[css3-flexbox] Make the line-wrapping algorithm clearer in the presence of negative-size items.

--HG--
extra : rebase_source : f22fcfe1da88181d86878395024e94a0d24514a4

Author: tabatkins

css3-flexbox/Overview.html

@@ -3191,11 +3191,13 @@ <h3 id=main-sizing><span class=secno>9.3. </span> Main Size Determination</h3>
       href="#single-line"><i>single-line</i></a>, collect all the flex items
       into a single flex line.
 
-     <li> Otherwise, starting from the first uncollected item, collect as
-      many consecutive flex items as will fit or until a forced break is
-      encountered (but collect at least one) into the flex container's inner
-      main size into a flex line. A break is forced wherever the CSS2.1
-      ‘<code class=css>page-break-before/after</code>’ <a href="#CSS21"
+     <li> Otherwise, starting from the first uncollected item, collect
+      consecutive items one by one until the first time that the
+      <em>next</em> collected item would not fit into the flex container's
+      inner main size, or until a forced break is encountered. If the very
+      first uncollected item wouldn't fit, collect just it into the line. A
+      break is forced wherever the CSS2.1 ‘<code
+      class=css>page-break-before/after</code>’ <a href="#CSS21"
       rel=biblioentry>[CSS21]<!--{{CSS21}}--></a> or the CSS3 ‘<code
       class=css>break-before/after</code>’ <a href="#CSS3-BREAK"
       rel=biblioentry>[CSS3-BREAK]<!--{{CSS3-BREAK}}--></a> properties

css3-flexbox/Overview.src.html

@@ -2464,11 +2464,13 @@ <h3 id='main-sizing'>
 
 				<li>
 					Otherwise,
-					starting from the first uncollected item, 
-					collect as many consecutive flex items as will fit
-					or until a forced break is encountered
-					(but collect at least one)
-					into the flex container's inner main size into a flex line.
+					starting from the first uncollected item,
+					collect consecutive items one by one 
+					until the first time that the <em>next</em> collected item 
+					would not fit into the flex container's inner main size,
+					or until a forced break is encountered.
+					If the very first uncollected item wouldn't fit,
+					collect just it into the line.
 					A break is forced wherever 
 					the CSS2.1 'page-break-before/after' [[CSS21]]
 					or the CSS3 'break-before/after' [[CSS3-BREAK]] properties