[css-overflow-3] Don't extend the overflow region to include boxes which are invisible because they're wholly outside the start edges of the scroll container. Fixes #2006

Author: fantasai

css-overflow-3/Overview.bs

@@ -188,7 +188,9 @@ Scrollable Overflow</h3>
 
 		<li>
 			the border boxes
-			of all boxes for which it is the containing block,
+			of all boxes for which it is the containing block
+			and whose border boxes are positioned not wholly outside
+			its <a>block-start</a> or <a>inline-start</a> padding edges,
 			accounting for transforms by projecting each box onto
 			the plane of the element that establishes its <a>3D rendering context</a>.
 			[[!CSS3-TRANSFORMS]]