[css-overflow][css-align] Require end-edge padding in scrollable overflow

See issue w3c#129 (comment)

Author: frivoal

css-align-3/Overview.bs

@@ -1122,33 +1122,13 @@ Overflow and Scroll Positions</h3>
 		and its content will appear to overflow its main-start edge.
 	</div>
 
-	Because the <a>scroll container</a>’s end-edge padding
-	is not always included in the <a>scrollable overflow area</a>
-	(see [[css-overflow-3#scrollable]]),
-	performing such alignment might require
-	that the <a>alignment subject</a> be positioned
-	in a way that would normally not be possible just by scrolling.
-	To resolve this problem
-	(and be consistent across alignment values),
-	all non-''align-content/normal'' values of 'align-content'/'justify-content'
-	cause the <a>scrollable overflow area</a> to be extended
-	in the relevant axis
-	however much would be required to make it possible to scroll
-	to a position satisfying ''align-content/end'' alignment in that axis.
-	(In general, this means that the <a>scroll container</a>’s padding
-	is added to the bounding rectangle of its in-flow children
-	on <em>both</em> sides of the <a>scrollable overflow area</a>.
-	See [[css-overflow-3]] for details.)
-
 	<figure>
-		<img src="images/scroll-align-padding.jpg" alt="">
+		<img src="images/scroll-align-padding.jpg">
 		<figcaption>
 			Issue: Replace this image with a proper SVG.
 		</figcaption>
 	</figure>
 
-	ISSUE: Coordinate wording with css-overflow once it's less of a mess.
-
 	None of this changes 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>.

css-align-3/images/scroll-align-padding.jpg

@@ -259,36 +259,31 @@ Scrollable Overflow</h3>
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 
 		<li>
-			Additional padding added as necessary
-			to the end-side of the [=scrollable overflow rectangle=]
-			such that the box’s content can be scrolled to a position
-			that would satisfy the requirements of ''place-content: end'' alignment.
-
-			Note: This padding represents the box’s own padding
-			within the [=scrollable overflow rectangle=]
+			Additional padding added
+			to the [=end=]-side of the [=scrollable overflow rectangle=]
+			as necessary to enable a scroll position
+			that satisfies the requirements of ''place-content: end'' alignment.
+
+			Note: This padding represents,
+			within the [=scrollable overflow rectangle=],
+			the box’s own padding
 			so that when its in-flow content is scrolled to the end,
-			there is padding between the end-edge of the content
+			there is padding between the end-edge of its content
 			and the border edge of the box.
 			It typically ends up being exactly the same size as the box's own padding,
-			except in a few cases, such as when an out-of-flow element,
+			except in a few cases--
+			such as when an out-of-flow element,
 			or the visible overflow of a descendent,
-			is increasing the size of the [=scrollable overflow rectangle=].
-
-			Including this padding is optional for <a>block containers</a>
-			in the inline axis if 'align-content'
-			is ''justify-content/normal''.
-
-			Note: It's hoped that including including inline-axis padding here is Web-compatible
-			as that is a better behavior,
-			but investigation as to whether that can be done without breaking existing content has not yet concluded.
-			This clause is optional while under investigation.
-			Existing behavior in the inline axis is
-			<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=8496">quirky</a>
-			<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=8499">and</a>
-			<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=9539">inconsistent</a>.
-			See <a href="https://github.com/w3c/csswg-drafts/issues/129">Issue 129</a>.
-
-			<!-- Make sure http://jsbin.com/qececaqilo/edit?html,css,output is clearly defined. -->
+			has already increased the size of the [=scrollable overflow rectangle=]
+			outside the conceptual “content edge” of the [=scroll container=]’s content.
+
+			<figure>
+				<img src="images/scroll-align-padding.jpg">
+				<figcaption>
+					Issue: Replace this image with a proper SVG.
+				</figcaption>
+			</figure>
+
 	</ul>
 
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