[css-scroll-snap] Clarify behavior of 'scroll-snap-stop'. Fixes #1552.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -744,23 +744,36 @@ Scroll Snap Limits: the 'scroll-snap-stop' property {#scroll-snap-stop}
     Media: interactive
     </pre>
 
-    This property specifies whether the <a>scroll container</a>
-    must stop at a <a>snap position</a> definited by this element,
-    or may pass through it before coming to rest.
+    When scrolling with an intended direction,
+    the <a>scroll container</a> can “pass over” several possible <a>snap positions</a>
+    (that would be valid to snap to,
+    if the scrolling operation used the same direction
+    but a lesser distance)
+    before reaching the natural endpoint of the scroll operation
+    and selecting its final <a>scroll position</a>.
+    The 'scroll-snap-stop' property allows such a possible <a>snap position</a>
+    to “trap” the scrolling operation,
+    forcing the <a>scroll container</a> to stop
+    before the scrolling operation would naturally end.
+
     Values are defined as follows:
 
     <dl dfn-type=value dfn-for=scroll-snap-stop>
         <dt><dfn>normal</dfn>
         <dd>
-            The <a>scroll container</a> may pass by a <a>snap position</a> defined
+            The <a>scroll container</a> may pass over a <a>snap position</a> defined
             by this element during the execution of a scrolling operation.
 
         <dt><dfn>always</dfn>
         <dd>
-            The <a>scroll container</a> must not pass by a <a>snap position</a>
-            defined by this element during the execution of a scrolling operation.
+            The <a>scroll container</a> must not pass over a <a>snap position</a>
+            defined by this element during the execution of a scrolling operation;
+            it must instead snap to the first of this element's <a>snap positions</a>.
     </dl>
 
+    This property has no effect on scrolling operations with only an <a>intended end position</a>,
+    as they do not conceptually “pass over” any <a>snap positions</a>.
+
 Snapping Mechanics {#snap-concepts}
 ===================================
 
@@ -1094,6 +1107,8 @@ Changes {#changes}
 				<ins><p class="note">This specification only applies to scrolling methods supported by the user agent;
 				it does not require the user agent to support any particular input or scrolling method.</p></ins>
 			</blockquote>
+		<li>Clarified the intended effects of 'scroll-snap-stop' on various scrolling operations.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/1552">Issue 1552</a>)
 		<li>Clarified that 'scroll-snap-stop' is applied to the <a>snap positions</a> defined by the element,
 			not applied to all <a>snap positions</a> in the <a>scroll snap container</a>.
 		<li>Fixed some syntax errors in examples and added a new one to the 'scroll-snap-type' section.