[css-grid-3] Simplified the sizing calcs for automatic-positioned subgrids in a grid lanes container. w3c#10926

Author: tabatkins

css-grid-3/Overview.bs

@@ -36,6 +36,7 @@ spec: css-grid-2;
 		text: collapsed grid track
 		text: min track sizing function
 		text: max track sizing function
+		text: parent grid
 	type: value; text: grid; for: display
 </pre>
 
@@ -582,6 +583,8 @@ Subgrids</h3>
 
 	A [=subgrid=] that is a [=grid lanes container=]
 	can be referred to as a <dfn export>grid lanes subgrid</dfn>.
+	To distinguish, a regular [=grid container=] [=subgrid=]
+	can be referred to as a <dfn export>regular grid subgrid</dfn>.
 
 <h3 id="repeat-notation">
 Track Repetition: the ''repeat()'' notation</h3>
@@ -670,7 +673,8 @@ Subgrid Item Contributions</h4>
 
 	When sizing the tracks of either a regular [=grid container=] or a [=grid lanes container=],
 	a [=grid lanes subgrid=] has special handling of items that have an [=automatic grid position=]:
-	*  Any such item is placed into every possible grid track
+
+	*  Any such item is placed into every possible [=parent grid=] track
 		that could be spanned by the [=grid lanes subgrid=].
 		(If the subgrid has a [=definite grid position=], thus only the spanned tracks;
 		if it has an [=automatic grid position=], then all tracks in the parent grid.)
@@ -679,6 +683,23 @@ Subgrid Item Contributions</h4>
 		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>.)
 
+	When sizing the tracks of a [=grid lanes container=],
+	a [=subgrid=] with an [=automatic grid position=]
+	(whether a [=regular grid subgrid=] or [=grid lanes subgrid=])
+	also has special handling of its items:
+
+	* <em>Every</em> item is placed into every possible [=parent grid=] track
+		that could be spanned by the [=subgrid=],
+		(ignoring any explicit placement of the item).
+	* Explicitly-placed sub-items only receive the subgrid's margin/border/padding contribution
+		when their explicit placement is at an edge of the subgrid
+		(as usual).
+
+	Note: Explicitly placed items inside an explicitly placed [=subgrid=]
+	only ever contribute to their actual [=parent grid=] tracks.
+	This is because in this case, just like in regular [=grid layout=],
+	the item's placement is known prior to track sizing.
+
 <h4 id="track-sizing-performance">
 Optimized Track Sizing</h4>
 
@@ -1318,10 +1339,12 @@ Additions Since Level 2</h3>
 <h3 class="no-num" id="recent-changes">
 Recent Changes</h3>
 
-	The following changes have been amde since the
+	The following changes have been made since the
 	<a href="https://www.w3.org/TR/2026/WD-css-grid-3-20260121/">21 January 2026 Working Draft</a>:
 	* Dropped the 'item-*' proposal for unified flow controls across layout modes.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/11480">Issue 11480</a>)
+	* Simplified the sizing calcs for automatic-positioned [=subgrids=] in a [=grid lanes container=].
+		(<a href="https://github.com/w3c/csswg-drafts/issues/10926">Issue 10926</a>)
 
 	The following changes have been made since the
 	<a href="https://www.w3.org/TR/2025/WD-css-grid-3-20251223/">23 December 2025 Working Draft</a>: