[css-overflow-3] Add issue about end-edge padding, allow UAs to experiment for feedback.

Author: fantasai

css-overflow-3/Overview.bs

@@ -149,8 +149,10 @@ Scrollable overflow</h3>
 	<ul>
 		<li>
 			the box’s own content and padding areas
+
 		<li>
 			all line boxes directly contained by the box
+
 		<li>
 			the border boxes
 			of all boxes for which it is the containing block,
@@ -173,6 +175,19 @@ Scrollable overflow</h3>
 			do not affect the <a>scrollable overflow region</a>.
 
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
+
+		<li>
+			Optionally,
+			additional padding on the end-edge sides,
+			corresponding to the end-side padding of the <a>scroll container</a>, such that the end edges of its in-flow content
+			coincide with the end edges of its content area
+			when scrolled to the end.
+
+			Issue: It's not yet clear if including the end-side padding in the scrollable layer is Web-compatible, so this clause is under investigation.
+			It appears that Chrome and Safari include such padding <a href="https://bug748518.bmoattachments.org/attachment.cgi?id=618078">in the block axis</a>;
+			and the behavior in the inline axis is <a href="http://jsbin.com/digadoyohi/1/edit?html,css,output">not</a> <a href="http://jsbin.com/vexahocayi/1/edit?html,css,output">clear</a>.
+
+			<!-- Make sure http://jsbin.com/qececaqilo/edit?html,css,output is clearly defined. -->
 	</ul>
 
 	The UA may <em>additionally</em> include