[css-align-3] Rewrite the scroll-container alignment section again, for both correctness and understandability. w3c#1425.

Author: tabatkins

css-align-3/Overview.bs

@@ -1104,23 +1104,46 @@ Content-Distribution Shorthand: the 'place-content' property</h3>
 <h3 id="overflow-scroll-position">
 Overflow and Scroll Positions</h3>
 
-	Content that overflows a <a>scroll container</a>
-	is aligned exactly like content in a non-scrollable container.
-	However, to prevent content from being shifted into the unscrollable region,
-	when the <a>content-distribution properties</a> are set on a <a>scroll container</a>,
-	the zero coordinate of its <a>scrollable overflow region</a>
-	(that is, the point beyond which content is unreachable)
-	is set to the top left corner of the <a>alignment subject</a>,
-	effectively changing the <a>initial scroll position</a>.
+	When the <a>content-distribution properties</a> are set
+	on a <a>scroll container</a> with overflowing content,
+	rather than shifting the position of its content,
+	they instead change its <a>initial scroll position</a>
+	so that the initially-visible content
+	of the <a>scroll container</a>
+	is the same as it would be if it were not a <a>scroll container</a>.
+	This does not change how scroll coordinates are assigned:
+	the origin is still where it would be in a ''align-content/start''-aligned container,
+	it just might be initially positioned outside the <a>scrollport</a>.
 
 	<div class='example'>
 		For example,
 		if a scrollable flex container is set to ''justify-content: flex-end''
 		(or ''justify-content: flex-start'' with ''flex-flow: row-reverse''),
-		it will be initially displayed scrolled all the way to the main-end edge of the scrollable area,
+		it will be initially displayed scrolled all the way to the main-end edge of its content,
 		and its content will appear to overflow its main-start edge.
 	</div>
 
+	Since performing such alignment can require that the <a>scrollable overflow region</a>
+	be extended by the amount of padding specified
+	on the end edges of the <a>scroll container</a>,
+	any non-''align-content/normal'' value of 'align-content' or 'justify-content'
+	will cause the <a>scrollable overflow region</a> to be extended
+	as would be required for ''align-content/end'' alignment
+	in the relevant axis.
+	(In general, this means that the <a>scroll container</a>’s padding
+	is added to the bounding rectangle of its in-flow children.
+	See [[css-overflow-3]] for details.)
+
+	ISSUE: Coordinate wording with css-overflow once it's less of a mess.
+
+	Note: The <a>alignment subject</a> is not necessarily
+	identical to the <a>scrollable overflow region</a>:
+	content overflowing the <a>alignment subject</a>,
+	such as an absolutely-positioned or out-of-flow element,
+	grows the <a>scrollable overflow region</a> but not the <a>alignment subject</a>,
+	thus an ''align-content/end''-aligned <a>scroll container</a>
+	might not be initially scrolled all the way to the bottom.
+
 <h3 id="baseline-align-content">
 Baseline Content-Alignment</h3>