[css-grid-3][masonry] Describe optimized track sizing algorithm. w3c#10266

Author: fantasai

css-grid-3/Overview.bs

@@ -632,6 +632,9 @@ 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
@@ -675,6 +678,51 @@ 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-performance">
+Optimized Track Sizing</h4>
+
+	Track sizing can be optimized by aggregating items
+	that have the same span size and placement
+	into a single virtual item
+	as follows:
+
+	<ol>
+		<li>
+			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 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
+			among the items in that group.
+
+			If the items apply [=baseline alignment=],
+			determine the baselines of the [=virtual masonry item=]
+			by placing all of its items into a single hypothetical grid track
+			and finding their shared baseline(s) and shims.
+			Increase the group's intrinsic size contributions accordingly.
+
+		<li>
+			Place hypothetical copies of each [=virtual masonry item=] into the [=grid axis=] tracks
+			in every position that the item could potentially occupy,
+			and run the [[css-grid-2#algo-track-sizing|track sizing algorithm]] with those items.
+			The resulting track sizes are the [=masonry container's=] track sizes.
+	</ol>
+
+	Note: This optimization should give the same results
+	as the track sizing description [[#track-sizing|above]];
+	if not this is an error, please
+	<a href="https://github.com/w3c/csswg-drafts/issues">report it to the CSSWG</a>.
+
 <h2 id="masonry-track-placement">
 Masonry Placement</h3>