Make endScrollOffset check inclusive for max-scroll-position (#37)

See the issue for more details

Fixes #19

Author: stephenmcgruer

Overview.bs

@@ -516,18 +516,30 @@ as follows:
 1.  If {{scrollSource}} is null, does not currently have a [=CSS layout box=], or
     if its layout box is not a [=scroll container=], return an unresolved time value.
 
-2.  Otherwise, let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
+1.  Otherwise, let <var>current scroll offset</var> be the current scroll offset of {{scrollSource}}
     in the direction specified by {{orientation}}.
 
-3.  If <var>current scroll offset</var> is less than {{startScrollOffset}}, return an unresolved
-    time value if {{fill}} is <code>none</code> or <code>forwards</code>, 
-    or 0 otherwise.
+1.  If <var>current scroll offset</var> is less than {{startScrollOffset}}, return an unresolved
+    time value if {{fill}} is <code>none</code> or <code>forwards</code>, or 0 otherwise.
 
-4.  If <var>current scroll offset</var> is greater than or equal to {{endScrollOffset}}, 
-    return an unresolved time value if {{fill}} is <code>none</code> or
-    <code>backwards</code>, or the [=effective time range=] otherwise.
+1.  If <var>current scroll offset</var> is greater than or equal to {{endScrollOffset}} then:
+    <div class="switch">
 
-5.  Return the result of evaluating the following expression:
+    : If {{endScrollOffset}} is less than the maximum scroll offset of {{scrollSource}} in
+        {{orientation}} and {{fill}} is <code>none</code> or <code>backwards</code>,
+
+    :: return an unresolved time value.
+
+    : Otherwise,
+
+    :: return the [=effective time range=].
+
+    </div>
+
+    Note: Checking for {{endScrollOffset}} being the maximum scroll offset ensures that the common
+    case of a 'whole scroller' ScrollTimeline does not become inactive when you scroll to the end.
+
+1.  Return the result of evaluating the following expression:
 
     <blockquote>
       <code>(<var>current scroll offset</var> - {{startScrollOffset}}) / ({{endScrollOffset}} - {{startScrollOffset}}) &times; [=effective time range=]</code>