[css-grid-3][masonry] Define handling of subgrids for track sizing. w3c#10053

Author: fantasai

css-grid-3/Overview.bs

@@ -570,6 +570,9 @@ Subgrids</h3>
 		results in the subgrid getting [=masonry layout=] in its [=inline axis=].
 	</div>
 
+	A [=subgrid=] that is a [=masonry container=]
+	can be referred to as a <dfn export>submasonry</dfn>.
+
 <h3 id="repeat-notation">
 Track Repetition: the ''repeat()'' notation</h3>
 
@@ -632,9 +635,6 @@ Grid Axis Track Sizing</h3>
 	* All items with an [=automatic grid position=] contribute
 		(regardless of whether they are ultimately placed in that track).
 
-	ISSUE(10053): Subgrids need special handling in order to avoid excessive performance costs.
-
-
 	<div class="example">
 		For example, suppose there are two columns in the [=grid axis=]
 		and that
@@ -678,6 +678,20 @@ Grid Axis Track Sizing</h3>
 	or all items are the same size
 	(or matching multiples of that size, in the case of spanning items).
 
+<h4 id="track-sizing-subgrid">
+Subgrid Item Contributions</h4>
+
+	When sizing the tracks of either a regular [=grid container=] or a [=masonry container=],
+	a [=submasonry=] has special handling of items that have an [=automatic grid position=]:
+	*  Any such item is placed into every possible grid track
+		that could be spanned by the [=submasonry=].
+		(If the submasonry has a [=definite grid position=], thus only the spanned tracks;
+		if it has an [=automatic grid position=], then all tracks in the parent grid.)
+	* Any such item receives the largest margin/border/padding contribution
+		of each edge at which it could hypothetically be placed.
+		If the item spans the entire subgrid, it receives both.
+		(See <a href="https://www.w3.org/TR/css-grid-2/#subgrid-item-contribution">CSS Grid Layout &sect;9</a>.)
+
 <h4 id="track-sizing-performance">
 Optimized Track Sizing</h4>
 
@@ -691,15 +705,13 @@ Optimized Track Sizing</h4>
 			Separate all the [=masonry items=] into <dfn>item groups</dfn>, according to the following properties:
 
 			* the span of the item
-			* the placement of the item, i.e. whether it an [=automatic grid position=], or a [=definite grid position=]--
-			and if so, what that definite position is
+			* the placement of the item,
+				i.e. which tracks it is allowed to be placed in
 			* the item's [=baseline-sharing group=]
 
 			Note: For example, an item with span 2 placed in the second track
 			will be in a different group than an item with span 2 that has an [=automatic grid position=].
 
-			ISSUE(10053): Subgrids need special handling in order to avoid excessive performance costs.
-
 		<li>
 			For each [=item group=], synthesize a <dfn>virtual masonry item</dfn>
 			that has the maximum of every intrinsic size contribution