[css-grid] Integrate 'stretch'-ing of tracks better into the track sizing algorithm. Fixes w3c#1150

Author: fantasai

css-grid/Overview.bs

@@ -3537,9 +3537,10 @@ Grid Sizing Algorithm</h3>
 			using the resulting size of the <a>grid</a> as its content size,
 			and the tracks are aligned within the <a>grid container</a>
 			according to the 'align-content' and 'justify-content' properties.
-			<span class="note">Note: This can introduce extra space within or between tracks.</span>
-			When introducing space within tracks,
-			only tracks with an ''grid-template-rows/auto'' <a>max track sizing function</a> accept space.
+
+			Note: This can introduce extra space between tracks,
+			potentially enlarging the grid area of any grid items spanning the gaps
+			beyond the space allotted to during track sizing.
 	</ol>
 
 	Once the size of each <a>grid area</a> is thus established,
@@ -3600,13 +3601,14 @@ Track Sizing Algorithm</h3>
 	and which will eventually be the track's final size,
 	and a <dfn>growth limit</dfn>,
 	a <<length>> which provides a desired maximum size for the <a>base size</a>.
-	There are 4 steps:
+	There are 5 steps:
 
 	<ol>
 		<li><a href="#algo-init">Initialize Track Sizes</a>
 		<li><a href="#algo-content">Resolve Intrinsic Track Sizes</a>
 		<li><a href="#algo-grow-tracks">Maximize Tracks</a>
-		<li><a href="#algo-flex-tracks">Stretch Flexible Tracks</a>
+		<li><a href="#algo-flex-tracks">Expand Flexible Tracks</a>
+		<li><a href="#algo-stretch">Expand Stretched <css>auto</css> Tracks</a>
 	</ol>
 
 <h3 id="algo-init">
@@ -3926,7 +3928,7 @@ Maximize Tracks</h3>
 	when it's sized to its <a property lt=max-width>max-width/height</a>.
 
 <h3 id="algo-flex-tracks">
-Stretch Flexible Tracks</h3>
+Expand Flexible Tracks</h3>
 
 	This step sizes flexible tracks
 	using the largest value it can assign to an ''fr''
@@ -4010,6 +4012,14 @@ Find the Size of an ''fr''</h4>
 			Return the <a>hypothetical fr size</a>.
 	</ol>
 
+<h3 id="algo-flex-tracks">
+Stretch <css>auto</css> Tracks</h3>
+
+	This step sizes expands tracks that have
+	an ''grid-template-columns/auto'' <a>max track sizing function</a>
+	by dividing any remaining positive, <a>definite</a> <a>free space</a>
+	equally amongst them.
+
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