[css-grid] Put explanation of the 'infinitely growable' flag into the spec.

Author: tabatkins

css-grid/Overview.bs

@@ -3071,7 +3071,44 @@ Resolve Intrinsic Track Sizes</h3>
 					Mark any tracks whose <a>growth limit</a> changed
 					from infinite to finite in this step
 					as <dfn>infinitely growable</dfn> for the next step.
-					<!-- http://lists.w3.org/Archives/Public/www-style/2014Mar/0500.html -->
+
+					<details class=why>
+						<summary>Why does the <a>infinitely growable</a> flag exist?</summary>
+
+						<a href="http://lists.w3.org/Archives/Public/www-style/2014Mar/0500.html">Peter Salas explains</a>:
+
+						<blockquote cite="http://lists.w3.org/Archives/Public/www-style/2014Mar/0500.html">
+							Consider the following case:
+
+							Two "auto" tracks (i.e. "minmax(min-content, max-content) minmax(min-content, max-content)").
+							Item 1 is in track 1, and has min-content = max-content = 10.
+							Item 2 spans tracks 1 and 2, and has min-content = 30, max-content = 100.
+
+							After resolving min-content/max-content for the first item, we have this.
+
+							track 1: used breadth = 10 max breadth = 10
+
+							track 2: used breadth = 0 max breadth = infinity
+
+							Then we resolve min-content/max-content for the second item.
+
+							Phase 1 sets the used breadth of track 2 to 20 so that the two tracks' used breadths sum to 30.
+							Phase 2 does nothing because there are no relevant tracks.
+							Phase 3 sets the max breadth of track 2 to 20 so that the two tracks' max breadths sum to 30.
+							In phase 4, we need to grow the sum of the max breadths by 70 to accommodate item 2.
+							Two options are:
+
+							1. Grow each track's max breadth equally,
+								and end up with max breadths = [45, 55].
+							2. Grow only the second track's max breadth,
+								and end up with max breadths = [10, 90].
+
+							By not considering the just-set max breadth as a constraint during space distribution
+							(i.e. by treating it as infinity),
+							we get the second result,
+							which we considered a better result because the first track remains sized exactly to the first item.
+						</blockquote>
+					</details>
 				<li>
 					<strong>For max-content maximums:</strong>
 					Lastly continue to increase the <a>growth limit</a> of tracks with

css-grid/issues-wd-20150108.html

@@ -99,13 +99,15 @@ <h1>CSS Grid Layout Level 1 Disposition of Comments for 2015-03-17 WD</h1>
 Comment:  <a href='https://lists.w3.org/Archives/Public/www-style/2015Jan/0506.html'>https://lists.w3.org/Archives/Public/www-style/2015Jan/0506.html</a>
 Response: <a href='https://lists.w3.org/Archives/Public/www-style/2015May/0110.html'>https://lists.w3.org/Archives/Public/www-style/2015May/0110.html</a>
 Closed:   Accepted</pre>
-<pre class=' open' id='issue-11'>
+<pre class='a' id='issue-11'>
 Issue 11. <a href='#issue-11'>#</a>
 Summary:  Add note to "infinitely growable" flag to help confusion
 From:     Sergio Villar Senin
 Comment:  <a href='https://lists.w3.org/Archives/Public/www-style/2015Feb/0013.html'>https://lists.w3.org/Archives/Public/www-style/2015Feb/0013.html</a>
 Response: <a href='https://lists.w3.org/Archives/Public/www-style/2015Feb/0014.html'>https://lists.w3.org/Archives/Public/www-style/2015Feb/0014.html</a>
-Open</pre>
+Response: <a href='https://lists.w3.org/Archives/Public/www-style/2015May/0111.html'>https://lists.w3.org/Archives/Public/www-style/2015May/0111.html</a>
+Closed:   Accepted
+Resolved: editorial</pre>
 <pre class=' open' id='issue-12'>
 Issue 12. <a href='#issue-12'>#</a>
 Summary:  Allow for centered grids of unknown number of columns

css-grid/issues-wd-20150108.txt

@@ -75,7 +75,9 @@ Summary:  Add note to "infinitely growable" flag to help confusion
 From:     Sergio Villar Senin
 Comment:  https://lists.w3.org/Archives/Public/www-style/2015Feb/0013.html
 Response: https://lists.w3.org/Archives/Public/www-style/2015Feb/0014.html
-Open
+Response: https://lists.w3.org/Archives/Public/www-style/2015May/0111.html
+Closed:   Accepted
+Resolved: editorial
 ----
 Issue 12.
 Summary:  Allow for centered grids of unknown number of columns