[css-grid-3][masonry] Make intrinsic track sizing handle spanning items w3c#8206 w3c#9041

Author: fantasai

css-grid-3/Overview.bs

@@ -175,6 +175,7 @@ Grid Axis Track Sizing</h3>
 	except that when considering which items contribute to intrinsic sizes:
 	* All items explicitly placed in that track contribute, and
 	* All items without an explicit placement contribute
+		(regardless of whether they are ultimately placed in that track).
 
 	<div class="example">
 		For example, suppose there are two columns in the [=grid axis=]
@@ -183,11 +184,31 @@ Grid Axis Track Sizing</h3>
 		* Item D is explicitly placed into the first column.
 
 		In this case, items A, B, C, and D all contribute to sizing the first column,
-		<em>and</em> A, B, and C (but not D) contribute to the second column.
+		while only A, B, and C (and not D) contribute to the second column.
 	</div>
 
 	In the case of spanning items with no explicit placement,
-	their size contribution is divided by their spanning size.
+	they are assumed to be placed at every possible start position,
+	and contribute accordingly.
+
+	<div class="example">
+		For example, suppose there are 5 columns in the [=grid axis=],
+		with the middle having a fixed size of ''100px''
+		and the other two being ''auto''-sized.
+		For the purpose of track sizing,
+		an item that spans 2 tracks
+		and has an intrinsic contribution of 220px
+		is essentially copied and assumed to exist:
+
+		* At grid line 1,
+			contributing 110px to each of the first two tracks.
+		* At grid line 2,
+			contributing 120px to the second track.
+		* At grid line 3,
+			contributing 120px to the fourth track.
+		* At grid line 4,
+			contributing 110px to the fourth and fifth tracks.
+	</div>
 
 	Note: This algorithm ensures that each track is at least big enough
 	to accommodate every item that is ultimately placed in it,