[css-overflow-3] Added issue about padding box and overflow.

Author: tabatkins

css-overflow-3/Overview.bs

@@ -155,6 +155,30 @@ Scrollable overflow</h3>
 		<li>
 			the box’s own content and padding areas
 
+			<div class=issue>
+				There's disagreement on the scrolling model.
+				2.1 apparently defined that you scrolled the *content* area;
+				the content would overflow the content-box,
+				and you would union that overflow with the content box to find the scrollable area.
+				In particular, this means that the content would be offset by the start-sides padding,
+				but if it overflowed,
+				it would go right to the edge on the end sides.
+				This is what Firefox and IE do.
+
+				At least some authors (and spec authors)
+				instead have the mental model that the padding box is what's scrollable,
+				so when you scroll to the end of the overflow,
+				there's the right/bottom padding.
+				Chrome/WebKit do this for the block axis, at least.
+				They're somewhat inconsistent for the inline axis;
+				there's something weird about how they handle lineboxes.
+
+				It seems that the block-axis padding is probably web-compatible to honor.
+				It's unclear that the inline-axis padding will be.
+				Further experimentation is needed.
+			</div>
+
+
 		<li>
 			all line boxes directly contained by the box