[web-animations-1] Restore compensatory seek to playbackRate setter

This fixes #2266.

web-animations-1/Overview.bs

@@ -1735,29 +1735,31 @@ is as follows:
 
 1.  Clear any [=pending playback rate=] on |animation|.
 
-1.  Set the [=playback rate=] of <a>animation</a> to <var>new playback
-    rate</var>.
+1.  Let <var>previous time</var> be the value of the
+    <a>current time</a> of <var>animation</var> before changing the
+    [=playback rate=].
 
-1.  Run the procedure to <a>update an animation's finished state</a> for
-    <var>animation</var> with the <var>did seek</var> flag set to true, and
-    the <var>synchronously notify</var> flag set to false.
+1.  Set the [=playback rate=] to <var>new playback rate</var>.
 
-    Note: We don't need to update the <a>previous current time</a> before
-    calling this since we set the <var>did seek</var> flag to true meaning we
-    will ignore the <a>previous current time</a>.
+1.  If <var>previous time</var> is <a lt="unresolved">resolved</a>,
+    <a>set the current time</a> of <var>animation</var> to
+    <var>previous time</var>
 
 <h5 id="performing-seamless-updates-to-the-playback-rate-of-an-animation">Performing seamless updates to the playback rate of an animation</h5>
 
-For an animation whose <a>current time</a> is not zero, the procedure to <a>set
-the playback rate</a> may result in a sharp change to the animation's
-current time causing the animation to jump.
+For an in-flight animation that is running on another process or thread, the
+procedure to <a>set the playback rate</a> may cause the animation to jump if the
+process or thread running the animation is not currently synchronized with the
+process or thread performing the update.
 
 In order to produce seamless changes to the [=playback rate=] of an
 [=animation=], animation's may have a <dfn>pending playback rate</dfn> that
 defines a playback rate to be applied after any necessary synchronization has
 taken place (for the case of animations running in a different thread or
 process).
 
+Initially the [=pending playback rate=] of an [=animation=] is unset.
+
 When an [=animation=], |animation|, is to <dfn>apply any pending playback
 rate</dfn> the following steps are performed:
 
@@ -1892,8 +1894,9 @@ condition from the following:
         <var>animation</var> is <a>unresolved</a> <em>and</em> it does
         <em>not</em> have a <a>pending play task</a>,
 ::  &rarr; <dfn lt="paused play state">paused</dfn>
-:   For <var>animation</var>, <a>current time</a> is <a>resolved</a> and
-    <em>either</em> of the following conditions are true:
+:   For <var>animation</var>, <a>current time</a> is <a
+    lt=unresolved>resolved</a> and <em>either</em> of the following conditions
+    are true:
     *   [=playback rate=] &gt; 0 and
         <a>current time</a> &ge; <a>target effect end</a>; <em>or</em>
     *   [=playback rate=] &lt; 0 and
@@ -4002,16 +4005,14 @@ interface Animation : EventTarget {
 
     <div class=note>
 
-    Setting this attribute is only intended for low-level synchronization
-    of animations.
-    It does <em>not</em> preserve the {{Animation/currentTime}} of the animation
-    nor make any attempt to synchronize with the playback state of animations
-    running on a separate process or thread.
+    Setting this attribute performs a synchronous update to the [=playback
+    rate=] meaning that it does not make any attempt to synchronize with the
+    playback state of animations running on a separate process or thread.
     As a result, setting the {{Animation/playbackRate}} for an in-flight
     animation may cause it to jump.
 
     To set the the [=playback rate=] for an in-flight animation such that it
-    continues to play from its current position, use {{updatePlaybackRate()}}.
+    smoothly updates, use the asynchronous {{updatePlaybackRate()}} method.
 
     </div>
 
@@ -4058,11 +4059,11 @@ interface Animation : EventTarget {
 ::  Suspends the playback of this animation by running the procedure to
     <a>pause an animation</a> for this object.
 :   <dfn method for=Animation lt='updatePlaybackRate(playbackRate)'>void updatePlaybackRate(playbackRate)</dfn>
-::  Updates the [=playback rate=] of this
-    animation such that it maintains its <a>current time</a> by performing the
-    <a>seamlessly update the playback rate</a> procedure, passing
-    {{Animation/updatePlaybackRate(playbackRate)/playbackRate}} as the <var>new
-    playback rate</var>.
+::  Performs an asynchronous update of the [=playback rate=] of this
+    animation by performing the [=seamlessly update the playback rate=]
+    procedure, passing
+    {{Animation/updatePlaybackRate(playbackRate)/playbackRate}} as the |new
+    playback rate|.
 
     <div class="parameters">
 
@@ -6085,11 +6086,9 @@ The following changes have been made since the <a
     (<a href="https://github.com/w3c/web-animations/issues/201">#201</a>).
 *   Dropped keyframe spacing, distance calculation, and retention of invalid
     keyframe property values.
-*   Made setting {{Animation/playbackRate}} not perform a compensatory seek
-    (i.e. not maintain the current time) and introduced
-    {{Animation/updatePlaybackRate(playbackRate)}} instead which maintains the
-    current time and synchronizes with animations running on another thread or
-    process.
+*   Introduced {{Animation/updatePlaybackRate(playbackRate)}} instead which
+    maintains the current time and synchronizes with animations running on
+    another thread or process.
 *   Added special handling to allow animating the 'offset' property from the
     programming interface using <code>cssOffset</code> to avoid conflict with
     the attribute name used to specify keyframe offsets.