[css-grid-3][masonry] Clarify dense packing algo w3c#9326

Author: fantasai

css-grid-3/Overview.bs

@@ -898,15 +898,22 @@ Masonry Layout and Placement Algorithm</h3>
 
 		<li>
 			If the [=masonry container=] uses ''item-pack/dense'' packing,
-			and there exists skipped empty space (e.g. due to spanning items)
-			in the layout into which the item, as it is sized now,
-			could have fit without impacting other items’ placement
-			if it were earlier in the placement order,
-			and the tracks it would have spanned at that point
-			have the same total used size as the tracks into which it is currently placed,
-			move the item into the earliest such position
-			and rewind the [=auto-placement cursor=] and the [=running position=]
+			and there exists skipped empty space in the layout
+			(e.g. due to spanning items)
+			into which the item, as it is sized now,
+			could have fit if it were placed earlier,
+			and where the spanned tracks have the same total used size
+			as the tracks into which it is currently placed,
+			then instead place it into the highest such space.
+			If there are multiple valid spaces within the [=tie threshold=] of the highest space,
+			place it in the [=start=]-most of them.
+			Rewind the [=auto-placement cursor=] and the [=running position=]
 			to their values before this item’s placement.
+
+			Note: Dense packing both ignores the [=auto-placement cursor=] when backfilling,
+			and does not update it after placement.
+			If there aren't any acceptable placement gaps to backfill, though,
+			it places items exactly as when ''item-pack/dense'' were not specified.
 	</ol>
 
 	Note: This algorithm chooses the track