Make the linebreaking algo handle forced breaks, per f2f resolution.

Author: tabatkins

css3-flexbox/Overview.html

@@ -2426,15 +2426,25 @@ <h3 id=main-sizing><span class=secno>9.3. </span> Main Size Determination</h3>
       single flexbox line.
 
      <li> Otherwise, starting from the first uncollected item, collect as
-      many consecutive flexbox items as will fit (but collect at least one)
-      into the flexbox's inner main size into a flexbox line. For this step,
-      the size of a flexbox item is its hypothetical outer main size, clamped
-      according to its min and max main size properties. Repeat until all
-      flexbox items have been collected into flexbox lines.
+      many consecutive flexbox items as will fit or until a forced break is
+      encountered (but collect at least one) into the flexbox's inner main
+      size into a flexbox 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
+      specify a fragmentation break.
+      <p> For this step, the size of a flexbox item is its hypothetical outer
+       main size, clamped according to its min and max main size properties.
+
+      <p> Repeat until all flexbox items have been collected into flexbox
+       lines.
+
       <p class=note> Note that items with zero main size will never start a
-       line unless they're the very first items in the flexbox. The "collect
-       as many" line will collect them onto the end of the previous line even
-       if the last non-zero item exactly "filled up" the line.
+       line unless they're the very first items in the flexbox, or they're
+       preceded by a forced break. The "collect as many" line will collect
+       them onto the end of the previous line even if the last non-zero item
+       exactly "filled up" the line.
     </ul>
 
    <li id=algo-flex> <strong><a href="#resolve-flexible-lengths">Resolve the

css3-flexbox/Overview.src.html

@@ -1677,17 +1677,27 @@ <h3 id='main-sizing'>
 				<li>
 					Otherwise,
 					starting from the first uncollected item, 
-					collect as many consecutive flexbox items as will fit 
-					(but collect at least one) 
+					collect as many consecutive flexbox items as will fit
+					or until a forced break is encountered
+					(but collect at least one)
 					into the flexbox's inner main size into a flexbox line.
-					For this step,
-					the size of a flexbox item is its hypothetical outer main size, 
-					clamped according to its min and max main size properties.
-					Repeat until all flexbox items have been collected into flexbox lines.
+					A break is forced wherever 
+					the CSS2.1 'page-break-before/after' [[CSS21]]
+					or the CSS3 'break-before/after' [[CSS3-BREAK]] properties 
+					specify a fragmentation break.
+
+					<p>
+						For this step,
+						the size of a flexbox item is its hypothetical outer main size, 
+						clamped according to its min and max main size properties.
+
+					<p>
+						Repeat until all flexbox items have been collected into flexbox lines.
 
 					<p class='note'>
 						Note that items with zero main size will never start a line
-						unless they're the very first items in the flexbox.
+						unless they're the very first items in the flexbox,
+						or they're preceded by a forced break.
 						The "collect as many" line will collect them
 						onto the end of the previous line
 						even if the last non-zero item exactly "filled up" the line.