[css-grid] Per f2f, auto lines are at padding edge and are used instead of adding new implicit lines for abspos. https://lists.w3.org/Archives/Public/www-style/2015May/0280.html

Author: tabatkins

css-grid/Overview.bs

@@ -2773,28 +2773,19 @@ With a Grid Container as Containing Block</h3>
 	contributes a special line to the <a>placement</a>
 	whose position is that of the corresponding padding edge of the <a>grid container</a>
 	(the padding edge of the scrollable area, if the <a>grid container</a> overflows).
+	These lines become the first and last lines (0th and -0th) of the <dfn>augmented grid</dfn>
+	used for positioning absolutely-positioned items.
 
 	Note: Thus, by default, the absolutely-positioned box's <a>containing block</a> will correspond
 	to the padding edges of the <a>grid container</a>, as it does for <a>block containers</a>.
 
-	<div class="issue">
-		A grid with visible overflow is problematic here, because it puts the last line of the grid after the padding edge.
-		Options are:
-		<ul>
-			<li>Use the grid edge instead of the padding edge when there's non-scrollable overflow.
-				This is inconsistent with how block containers work.
-			<li>Deal with negative-width grid tracks resulting from this inversion.
-			<li>Use the grid edge, but then clamp the containing block to the padding area.
-			<li>Honor the non-''auto'' edge and clamp the size to zero.
-			<li>Something else?
-		</ul>
-	</div>
-
 	Absolute positioning occurs after layout of the <a>grid</a>	and its in-flow contents
-	and does not contribute to the sizing of any grid tracks.
-	Any <em>new</em> <a>implicit grid lines</a> added by a <a>grid-placement property</a>
-	are positioned atop the last <a>grid line</a> on that side of the <a>grid</a>
-	(rather than having their placement determined by 'grid-auto-rows'/'grid-auto-columns' and the <a>grid sizing algorithm</a>).
+	and does not contribute to the sizing of any grid tracks
+	or affect the size/configuration of the grid in any way.
+	If a <a>grid-placement property</a> refers to a non-existent line
+	either by explicitly specifying such a line or by spanning outside of the existing <a>implicit grid</a>,
+	it is instead treated as specifying ''auto''
+	(instead of creating new <a>implicit grid lines</a>).
 
 	If the <a>placement</a> only contains a <a>grid span</a>,
 	replace it with the two ''auto'' lines in that axis.