[css-grid-3] Specify how large negative margins (don't) affect auto-placement. w3c#12918

Author: tabatkins

css-grid-3/Overview.bs

@@ -896,6 +896,15 @@ Masonry Layout and Placement Algorithm</h3>
 			Set the [=running position=] of the spanned  [=grid axis=] tracks
 			to <code><var>max_pos</var> + [=outer size=] + 'grid-gap'</code>.
 
+			For this purpose, the [=outer sizes=] of the box are floored at zero.
+
+			Note: That is, if the box has sufficiently large negative margins,
+			it won't somehow count as a negative <em>size</em> here.
+			The [=running position=] of a track never decreases,
+			so a negative-sized box
+			won't cause future normally-sized items placed in the track
+			to overlap previous items.
+
 		<li>
 			If the [=masonry container=] uses ''item-pack/dense'' packing,
 			and there exists skipped empty space in the layout
@@ -910,6 +919,12 @@ Masonry Layout and Placement Algorithm</h3>
 			Rewind the [=auto-placement cursor=] and the [=running position=]
 			to their values before this item’s placement.
 
+			Items that <em>visually</em> intrude into preceding empty spaces
+			(via negative margins, ''position: relative'', transforms, etc.),
+			do not affect the size of those empty spaces.
+			<span class=note>Later items might get placed in those spaces
+			and visually overlap these previous items.</span>
+
 			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,