[css-grid-1][masonry] Make intrinsic sizing not depend on placement #…

…8206 #9041

css-grid-3/Overview.bs

@@ -168,51 +168,36 @@ Line Name Resolution</h3>
 	The [[css-grid-2#line-placement|line name resolution]]
 	works exactly is in <a href="https://www.w3.org/TR/css-grid/">CSS Grid</a>.
 
-<h3 id="grid-item-placement">
-Grid Item Placement</h3>
-
-	[=Grid items=] are placed using these steps:
-
-	<ol>
-		<li>
-			Place the items as if the [=masonry axis=]
-			had ''grid-template-columns/none'' specified in that axis.
+<h3 id="track-sizing">
+Grid Axis Track Sizing</h3>
 
-		<li>
-			For any items that were placed at the first (hypothetical) implicit line
-			in the [=masonry axis=] in step 1,
-			those items keep their placement in both axes as their final placement.
-			They will be laid out first in each [=grid axis=] track
-			and their [=grid axis=] placement is considered definite henceforth.
-			All other items ignore their placement from step 1.
-			Any item with a specified definite placement in the [=masonry axis=]
-			that doesn't result in it being placed at the first implicit line
-			in the [=masonry axis=]
-			will be treated as having ''grid-area/auto''-placement in that axis.
+	Track sizing works the same as in [[css-grid-2#algo-track-sizing|CSS Grid]],
+	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
 
-		<li>
-			Place items using the [[#masonry-layout-algorithm|Masonry layout algorithm]] below.
-	</ol>
+	<div class="example">
+		For example, suppose there are two columns in the [=grid axis=]
+		and that
+		* Items A, B, and C have no explicit placement.
+		* Item D is explicitly placed into the first column.
 
-	(The reason for step 1 above is to determine which items contribute
-	to intrinsic track sizing in the [=grid axis=];
-	see [[#track-sizing]] below.)
+		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.
+	</div>
 
-<h3 id="track-sizing">
-Grid Axis Track Sizing</h3>
+	In the case of spanning items with no explicit placement,
+	their size contribution is divided by their spanning size.
 
-	Track sizing works the same as in [[css-grid-2#algo-track-sizing|CSS Grid]],
-	except that that the algorithm is run only with
-	the subset of items that have a known [=grid axis=] placement.
-	This subset contains the items placed at the first implicit line in the [=masonry axis=]
-	in <a href="#grid-item-placement">Grid Item Placement</a> above,
-	plus the items that have a specified definite placement in the [=grid axis=],
-	plus the items that span all [=grid axis=] tracks
-	(which obviously will be placed at the first implicit line
-	since there is no other way to place them).
-	Other items will be placed in the [=grid axis=] by the [[#masonry-layout-algorithm]] below
-	and don't contribute to intrinsic track sizing
-	(because that would create circular dependencies).
+	Note: This algorithm ensures that each track is at least big enough
+	to accommodate every item that is ultimately placed in it,
+	and does not create dependency cycles between placement and track sizing.
+	However, depending on the variation in sizes,
+	tracks could be larger than necessary:
+	an exact fit is only guaranteed if
+	all items are explicitly placed in the [=grid axis=]
+	or all items are the same size
+	(or matching multiples of that size, in the case of spanning items).
 
 <h4 id="repeat-auto-fit">
 repeat(auto-fit)</h3>
@@ -234,60 +219,39 @@ Masonry Layout Algorithm</h3>
 	Items are placed in [=order-modified document order=],
 	but items with a definite placement are placed
 	before items with an indefinite position (as in regular grid layout).
+
 	For each of the tracks in the [=grid axis=],
 	keep a <dfn>running position</dfn> initialized to zero.
-	For each item placed at the first implicit line in the [=masonry axis=]
-	in <a href="#grid-item-placement">Grid Item Placement</a> step 1 above:
-
+	First for each item with a definite placement in the [=grid axis=],
+	then for each item with an indefinite placement:
 	<ol>
 		<li>
-			Position the item at the [=content edge=] of the [=grid container=] in the [=masonry axis=]
-			and at its start track in the [=grid axis=].
-
-		<li>
-			Calculate the size of the <a href="#containing-block">containing block</a>
-			and then layout the item.
-			Then, calculate its resulting [=margin box=] in the [=masonry axis=].
-			Set the [=running position=] of the [=grid axis=] tracks the item spans
-			to the maximum of <code>margin-box-end + grid-gap</code>
-			and the current [=running position=] of those tracks.
-	</ol>
-
-	Note: This means that items with a definite placement at line 1 in the [=masonry axis=]
-	by <a href="#grid-item-placement">Grid Item Placement</a> step 1
-	can be made to intentionally overlap.
-
-	The remaining items get their final placement using the following steps:
-	<ol>
-		<li>
-			If the item has an indefinite placement in the [=grid axis=],
-			then resolve its definite placement in the [=grid axis=] using these substeps:
+			If the item has an definite placement in the [=grid axis=],
+			use that placement.
+			Otherwise, resolve its [=grid axis=] placement using these substeps:
 
 			<ol>
-				<li>Starting at the first [=grid axis=] line in the implicit grid...
+				<li>Starting at the first [=grid axis=] line in the [=implicit grid=].
 				<li>Find the largest [=running position=] of the [=grid axis=] tracks
 					that the item would span if it were placed at this line,
-					and call this position <code>max_pos</code>.
+					and call this position <var>max_pos</var>.
 				<li>Increment the line number and repeat step 2
 					until the item would no longer fit inside the grid.
-				<li>Pick the line that resulted in the smallest <code>max_pos</code>
+				<li>Pick the line that resulted in the smallest <var>max_pos</var>
 					as the item's definite placement in the [=grid axis=].
 			</ol>
 
-			Otherwise, a definite placement in the [=grid axis=] is honored
-			the same as in a regular grid.
-
 		<li>
-				Position the item at its [=grid axis=] start track
-				and the maximum of the [=running position=]s
-				of the tracks it spans.
+			Place the item in its [=grid axis=] tracks
+			at the maximum of the [=running position=]s
+			of the tracks it spans.
 
 		<li>
 			Calculate the size of the item's <a href="#containing-block">containing block</a>
 			and then layout the item.
 			Then calculate its resulting [=margin box=] in the [=masonry axis=].
-			Set the [=running position=] of the [=grid axis=] tracks the item spans
-			to <code>max_pos + margin-box-end + grid-gap</code>.
+			Set the [=running position=] of the spanned  [=grid axis=] tracks
+			to <code><var>max_pos</var> + <var>margin-box-end</var> + <var>grid-gap</var></code>.
 	</ol>
 
 <h3 id="masonry-auto-flow">
@@ -363,10 +327,7 @@ The Implicit Grid</h2>
 	The [=implicit grid=] is formed in the same way as for a regular [=grid container=].
 	However, it's only used in the [=grid axis=].
 	The flow axis specified by ''grid-auto-flow'' is ignored:
-	items are always placed by filling the [=grid axis=]
-	(though note that the ''grid-auto-flow'' property's ''grid-auto-flow/dense'' keyword
-	does have an effect in determining which items end up at line 1 in the [=masonry axis=],
-	in [[#grid-item-placement]] step 1).
+	items are always placed by filling the [=grid axis=].
 	''direction:rtl'' reverses the grid if the [=inline axis=] is the [=grid axis=]
 	(as usual for a regular [=grid container=])
 	and it makes items flow from right to left if the [=inline axis=] is the [=masonry axis=].