[css-grid-3][masonry] Give auto-fit distinct behavior in masonry. #10274

Author: fantasai

css-grid-3/Overview.bs

@@ -602,16 +602,26 @@ repeat(auto-areas)</h3>
 <h4 id="repeat-auto-fit">
 repeat(auto-fit)</h3>
 
-	''grid-template-rows/repeat(auto-fit)'' behaves as ''grid-template-rows/repeat(auto-fill)''
-	when the other axis is a [=masonry axis=].
-	The reason for this is that ''grid-row/auto''-placed items depend
-	on the layout size of their siblings.
-	Removing empty tracks after layout wouldn't be possible in most cases
-	since it might affect any intrinsic track sizes.
-	Even if all track sizes are definite,
-	the containing block size could change for grid-aligned absolutely-positioned descendants.
-	This makes ''grid-template-rows/repeat(auto-fit)''
-	impossible to support in a [=grid container=] with [=masonry layout=].
+	In [=masonry containers=] (as in regular [=grid containers=])
+	''repeat()/auto-fit()'' acts like ''repeat()/auto-fill'',
+	but with empty tracks [=collapsed tracks|collapsed=].
+	However, because placement occurs after track sizing,
+	[=masonry containers=] use a heuristic
+	to determine if a track will be occupied:
+
+	* All tracks occupied by explicitly placed items are considered occupied.
+	* With the sum of the spans of all auto-placed items as <var>N</var>,
+		all unoccupied tracks up to the <var>N</var>th such track
+		are considered occupied.
+
+	All tracks produced by the ''repeat()/auto-fit'' repetition and considered unoccupied by this heuristic
+	are assumed “empty” and are [=collapsed tracks|collapsed=].
+	A [=collapsed track=] cannot accept placement of auto-placed items.
+
+	Note: It is possible for an auto-placed item to be placed in a track when ''repeat()/auto-fill'' is used
+	that would be collapsed if ''repeat()/auto-fit'' is used
+	if there are auto-placed items with a span greater than 1
+	mixed with explicitly-placed items that leave gaps too small for the auto-placed items.
 
 <h3 id="track-sizing">
 Grid Axis Track Sizing</h3>