[css-overflow-3] Define that end padding is included in the scrollable overflow area except for block containers. Trying again because the last commit was wrong and bad. This time working off https://www.w3.org/TR/2018/WD-css-align-3-20181206/#overflow-scroll-position as a model. #3665

Author: fantasai

css-overflow-3/Overview.bs

@@ -238,12 +238,14 @@ Scrollable Overflow</h3>
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 
 		<li>
-			Padding on the end-edge sides of the <a>scrollable overflow area</a>
+			Padding on the end-edge sides of the <a>scrollable overflow rectangle</a>
 			representing the end-side padding applied to the <a>scroll container</a>,
-			added such that the end edges of its <a>in-flow</a> content
-			coincide with the end edges of the <a>scroll container</a>’s <a>content box</a>
-			when its contents have been scrolled to the end.
-			Including this padding is optional for <a>block containers</a>.
+			added such that its content can be scrolled to a position
+			that would satisfy the requirements of ''place-content: end'' alignment.
+			Including this padding is optional for <a>block containers</a>
+			in any axis whose corresponding <a>content-distribution property</a>
+			('align-content'/'justify-content')
+			is ''align-content/normal''.
 
 			Issue(129): It's not yet clear if including the end-side padding in the scrollable layer of block containers 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>;