Update procedure for determining if at a progress timeline boundary (#13819)

Update spec based on discussion in #12134. Keeping the animation in effect at a phase boundary is intended to alleviate the need for a fill mode, when the scroll range for the animation aligns with the maximum or minimum scroll position. The update aligns with the original intent of the feature.

Author: kevers-google

web-animations-2/Overview.bs

@@ -215,10 +215,28 @@ Add:
 > (e.g. scroll) timeline, the duration has a fixed upper bound. In this
 > case, the timeline is a <dfn lt="progress-based timeline">progress-based
 > timeline</dfn>, and its [=timeline duration=] is 100%.
+>
+> <dfn>minimum timeline time</dfn>
+>
+> For a [=progress-based timeline=], the minimum timeline time is the minimum
+> value that can be produced for the timeline. For a
+> [=monotonically increasing timeline=], minimum timeline time is -infinity.
+>
+> <dfn>maximum timeline time</dfn>
+>
+> For a [=progress-based timeline=], the maximum timeline time is the maximum
+> possible value producible for timeline time, and matches the
+> [=timeline duration=]. For a [=monotonically increasing timeline=]. it is
+> +infinity.
+>
+> Note: The minimum and maximum values will be the values associated with the
+> minimum and maximum scroll offsets, respectively, when using a scroll
+> timeline.
 
 <h3 id="animation-frame-loop">Animation Frames</h3>
 
-Update the note on the first step of the algorithm by adding the following bullet:
+Update the note on the first step of the algorithm by adding the following
+bullet:
 
 > *   Updating the [=animation trigger state|state=] of [=animation triggers=]
 >     by running the [=updating animation trigger state=] procedure.
@@ -1241,55 +1259,34 @@ condition:
 Add the following definitions to the list used in determining the phase of
 an <a>animation effect</a>.
 
-:   <dfn>at progress timeline boundary</dfn>
-::  Determined using the following procedure:
-
-    1.  If <em>any</em> of the following conditions are true:
-        *   the associated <a>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
+:   <dfn>at timeline boundary</dfn>
+::  Determined based on the first matching condition:
 
-    1.  Let <var>effective start time</var> be the <a>animation</a>'s
-        [=animation/start time=] if resolved, or zero otherwise.
+    <dl class="switch">
+        :   If the [=minimum timeline time=] matches the
+            [=timeline current time=]:
 
-    1.  Set <var>unlimited current time</var> based on the first matching
-        condition:
+        ::  Return true
 
-        :   [=animation/start time=] is resolved:
+        :   If the [=maximum timeline time=] matches the
+            [=timeline current time=]:
 
-        ::  <code>(<var>timeline time</var> - [=animation/start time=])
-            &times; [=playback rate=]</code>
+        ::  Return true
 
         :   Otherwise
 
-        ::  <code>animation's [=animation/current time=]</code>
-
-    1.  Let <var>effective timeline time</var> be
-        <code>|unlimited current time| / <a>animation</a>'s
-        [=playback rate=] + |effective start time|</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.
-
-Issue: This procedure can likely be simplified, and instead determine if at a
-scrolling boundary regardless of playback rate or start time. The surprising
-behavior that this is trying to prevent is an animation becoming inactive
-precisely at the scroll limit, alleviating the need for set a fill-mode with
-a ScrollTimeline. Checking if timeline [=timeline/current time=] is
-0 or timeline duration may be sufficient.
+        ::  Return false
+    </dl>
 
+    Note: The intent of this procedure for use in phase determination is to
+    avoid the need for fill-mode when using a scroll-timeline where the active
+    timeline range spans the full scroll range, or a view-timeline where the
+    subject is aligned with the start or end of the source scroll container.
+    This algorithm is not intended to handle the general case, but rather
+    avoids complications, such as when a view timeline subject  that is not
+    aligned with either bound of the scroll container being a candidate when the
+    magnitude of the playbackRate > 1. The algorithm also doesn't handle paused
+    animations. A more general solution is potentially feasible.
 
 Replace:
 
@@ -1312,7 +1309,7 @@ with:
 >     <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>  and not
->     <a>at progress timeline boundary</a>.
+>     <a>at timeline boundary</a>.
 
 
 Replace:
@@ -1336,7 +1333,7 @@ with:
 >     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> and not
->     <a>at progress timeline boundary</a>.
+>     <a>at timeline boundary</a>.
 
 
 <h4 id="fill-modes">Fill modes</h4>