[css-overflow-3] More precisely define impact of scrollbars on layout (and backgrounds). w3c#3348

Author: fantasai

css-overflow-3/Overview.bs

@@ -484,12 +484,15 @@ Overflow in Print and Other Static Media</h3>
 <h3 id="scrollbar-layout">
 Scrollbars and Layout</h3>
 
-	In the case of a scrollbar being placed on an edge of the element's box,
-	it should be inserted between the inner border edge
+	When reserving space for a scrollar placed at the edge of an element's box,
+	the reserved space is inserted between the inner border edge
 	and the outer padding edge.
-	Any space taken up by the scrollbars should be
-	taken out of (subtracted from the dimensions of)
-	the containing block formed by the element with the scrollbars.
+	For the purpose of the [=background positioning area=] and [=background painting area=], however,
+	this reserved space is considered to be part of the [=padding box=].
+
+	When the box is sized to fit its contents,
+	this reserved space is added to the size of its contents.
+	It is otherwise subtracted from space alotted to the [=content area=].
 
 	Issue: import examples from [[CSS3-BOX]].