[css-scroll-snap] Specify that scroll containers automatically trap snap points. Clarify that this works on the containing-block chain. Add an issue for trapping only one axis.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -9,7 +9,7 @@ ED: https://drafts.csswg.org/css-scroll-snap/
 Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
 Abstract: A brief description of an alternate model for scroll-snapping.
-Ignored Terms: snap position, snap positions, inertial scroll, inertial scrolls, semantic scroll, semantic scrolls, scrollable area, scroll-group-align
+Ignored Terms: snap position, snap positions, inertial scroll, inertial scrolls, semantic scroll, semantic scrolls, scrollable area, scroll-group-align, containing block chain
 </pre>
 
 <pre class="link-defaults">
@@ -220,6 +220,13 @@ Scroll Snapping Model {#snap-model}
 	as coordinates of the <a>scrollable area</a>
 	with the 'scroll-snap-points-x' and 'scroll-snap-points-y' properties.
 
+	<a>Snap positions</a> must only affect the nearest ancestor
+	(on the element's <a>containing block chain</a>)
+	<a>scroll container</a>.
+
+	Issue: Or should a <a>scroll container</a> be allowed to process only one axis-worth of <a>snap positions</a>,
+	leaving the rest for a <a>scroll container</a> higher in the tree?
+
 Scroll Snapping Container {#snap-container}
 =========================
 
@@ -296,7 +303,7 @@ Scroll Snapping Rules: the 'scroll-snap-type' property {#snap-type}
 		<dd>
 			Identical to ''scroll-snap-type/none'',
 			except that if this is specified on a non-<a>scroll container</a>,
-			it "traps" all <a>snap positions</a> from its descendants,
+			it "traps" all <a>snap positions</a> from its <a>containing block chain</a> descendants,
 			preventing them from propagating up to an ancestor <a>scroll container</a>.
 
 			Issue: This is proposed to resolve concerns