[css-grid-2] Define handling of subgrid mbp if edge-adjacent tracks are empty. <https://lists.w3.org/Archives/Public/www-style/2018Jul/0028.html> w3c#2592

Author: fantasai

css-grid-2/Overview.bs

@@ -201,12 +201,9 @@ Characteristics of a Subgrid Item</h3>
 
 		<li id="subgrid-size-contribution">
 			The <a>subgrid</a> itself lays out as an ordinary <a>grid item</a> in its parent grid,
-			but acts as if it was completely empty for sizing purposes
+			but acts as if it was completely empty for track sizing purposes
 			in the subgridded dimension.
 
-			ISSUE: Correct handling of the subgrid’s margins/border/padding
-			is an open issue, see <a href="https://github.com/w3c/csswg-drafts/issues/2592">discussion</a>.
-
 		<li id="subgrid-item-contribution">
 			The <a>subgrid</a>'s own <a>grid items</a> participate
 			in the sizing of its parent grid in the subgridded dimension(s)
@@ -369,6 +366,27 @@ Characteristics of a Subgrid Item</h3>
 				</dl>
 			</div>
 
+		<li id="subgrid-edge-placeholders">
+			For each edge of a non-empty subgrid,
+			to account for the subgrid’s margin/border/padding at that edge,
+			a hypothetical item is contributed to the track sizing algorithm.
+			This item’s sizes are taken from the sizes of the largest item
+			with the smallest span
+			from the occupied track closest to that edge of the subgrid,
+			and are additionally inflated by the subgrid’s own margin/border/padding
+			at that edge (or both edges,
+			if it happens to be the most extreme item on both sides).
+			Similarly, the hypothetical item’s span is taken
+			from that same real item’s span,
+			and inflated by the number of empty tracks between it
+			and the relevant subgrid’s edge(s).
+
+			Note: This step can be shortcut if
+			the tracks closest to the subgrid’s edges contain real items,
+			which would have already
+			<a href="#subgrid-item-contribution">accounted for the subgrid’s margin/border/padding</a>
+			as described above.
+
 		<li id="subgrid-box-alignment">
 			The <a>subgrid</a> is always stretched in its subgridded dimension(s:
 			the 'align-self'/'justify-self' properties on it are ignored,
@@ -700,6 +718,17 @@ Changes</h2>
 
 <h2 class="no-num" id="changes">Changes</h2>
 
+	<h3 class="no-num" id="changes-20180628">
+	Changes since the
+	<a href="https://www.w3.org/TR/2018/WD-css-grid-2-20180628/">June 2018
+	CSS Grid Layout Level 2 Working Draft</a></h3>
+
+	<ul>
+		<li>Defined handling of the subgrid’s margins/borders/padding
+		when the track closest to its edge is empty.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/2592">Issue 2592</a>)
+	</ul>
+
 	<h3 class="no-num" id="changes-20180427">
 	Changes since the
 	<a href="https://www.w3.org/TR/2018/WD-css-grid-2-20180427/">April 2018