[web-animations-2] Make active boundary times inclusive if animation associated with a progress-based timeline (w3c#6702)

Addresses issue w3c#5223 by changing before and after phase calculations to have explicit exceptions for progress based timelines to make an active boundary end point inclusive at the timeline end.

Author: kevers-google

web-animations-2/Overview.bs

@@ -1066,13 +1066,43 @@ condition:
 > *   the <a>animation effect</a> has a <a>parent group</a> and the
 >     <a>parent group</a> is <a>current</a>.
 
-Add the following definition to the list used in determining the phase of
+Add the following definitions to the list used in determining the phase of
 an <a>animation effect</a>.
 
 :   <dfn>effective timeline phase</dfn>
 ::  The associated <a>animation</a>'s <a>hold phase</a> if resolved, otherwise
     the <a>animation</a> timeline's [=timeline phase=].
 
+:   <dfn>at progress timeline boundary</dfn>
+::  Determined using the following procedure:
+
+    1.  If <em>any</em> of the following conditions are true:
+        *   the associated <an>animation</a>'s timeline is not a
+            [=progress-based timeline=], or
+        *   the associated <a>animation</a>'s [=timeline duration=] is
+            unresolved or zero, or
+        *   the <a>animation</a>'s [=playback rate=] is zero
+
+        return false
+
+    1.  Let <var>effective start time</var> be the <a>animation</a>'s
+        [=start time=] if resolved, or zero otherwise.
+
+    1.  Let <var>effective timeline time</var> be
+        <code><a>animation</a>'s <a>current time</a> / <a>animation</a>'s
+        [=playback rate=] + <var>effective start time</var></code>
+
+    1.  Let <var>effective timeline progress</var> be
+        <code><var>effective timeline time</var> / [=timeline duration=]</code>
+
+    1.  If <var>effective timeline progress</var> is 0 or 1, return true,
+        otherwise false.
+
+Issue: This procedure is not strictly correct for a paused
+animation if the <a>animation</a>'s current time is explicitly set, as this can
+introduce a lead or lag, between the <a>timeline<a>'s current time and
+<a>animation</a>'s current time.
+
 Replace:
 
 > An <a>animation effect</a> is in the before phase if the animation effect's
@@ -1096,8 +1126,9 @@ with:
 >     <em>and</em> <em>either</em> of the following conditions are met:
 >     1.  the <a>local time</a> is less than the <a>before-active boundary
 >         time</a>, <em>or</em>
->     1.  the <a>animation direction</a> is "backwards" and the
->         <a>local time</a> is equal to the <a>before-active boundary time</a>.
+>     1.  The <a>animation direction</a> is "backwards", the <a>local time</a>
+>         is equal to the <a>before-active boundary time</a> and not
+>         <a>at progress timeline boundary</a>.
 
 Replace:
 
@@ -1122,8 +1153,9 @@ with:
 >     <em>and</em> <em>either</em> of the following conditions are met:
 >     1.  the <a>local time</a> is greater than the <a>active-after boundary
 >         time</a>, <em>or</em>
->     1.  the <a>animation direction</a> is "forwards" and the <a>local time</a>
->         is equal to the <a>active-after boundary time</a>.
+>     1.  the <a>animation direction</a> is "forwards", the <a>local time</a>
+>         is equal to the <a>active-after boundary time</a> and not
+>         <a>at progress timeline boundary</a>.
 
 <h4 id="fill-modes">Fill modes</h4>