Integrate with the Paint Timing spec

This eliminates Paint Timing's monkey patching, and fixes some minor
issues with when we mark paint timing. It does so by introducing the new
concept of "rendering opportunity", which roughly maps to frame rate,
but in a model-agnostic way.

This also lays the groundwork necessary to correctly report event
duration for the Event Timing spec, per the issue at
w3c/event-timing#6.

Author: tdresser

source

@@ -2880,6 +2880,12 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://w3c.github.io/webappsec-mixed-content/#a-priori-authenticated-url"><i>a priori</i> authenticated URL</dfn></li>
     </ul>
 
+    <p>The following terms are defined in <cite>Paint Timing</cite>: <ref spec=PAINTTIMING></p>
+
+    <ul class="brief">
+     <li><dfn data-x-href="https://w3c.github.io/paint-timing/#mark-paint-timing">mark paint timing</dfn></li>
+    </ul>
+
    </dd>
 
 
@@ -89134,45 +89140,63 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
      </li>
 
      <li>
-      <p>If there are <span data-x="top-level browsing context">top-level browsing contexts</span>
-      <var>B</var> that the user agent believes would not benefit from having their rendering
-      updated at this time, then remove from <var>docs</var> all <code>Document</code> objects whose
-      <span data-x="concept-document-bc">browsing context</span>'s <span>top-level browsing
-      context</span> is in <var>B</var>.</p>
-
-      <div class="note">
-       <p>Whether a <span>top-level browsing context</span> would benefit from having its rendering
-       updated depends on various factors, such as the update frequency. For example, if the browser
-       is attempting to achieve a 60Hz refresh rate, then these steps are only necessary every 60th
-       of a second (about 16.7ms). If the browser finds that a <span>top-level browsing
-       context</span> is not able to sustain this rate, it might drop to a more sustainable 30Hz for
-       that set of <code>Document</code>s, rather than occasionally dropping frames. (This
-       specification does not mandate any particular model for when to update the rendering.)
-       Similarly, if a <span>top-level browsing context</span> is in the background, the user agent
-       might decide to drop that page to a much slower 4Hz, or even less.</p>
-
-       <p>Another example of why a browser might skip updating the rendering is to ensure certain
-       <span data-x="concept-task">tasks</span> are executed immediately after each other, with only
-       <span data-x="perform a microtask checkpoint">microtask checkpoints</span> interleaved (and
-       without, e.g., <span data-x="run the animation frame callbacks">animation frame
-       callbacks</span> interleaved). For example, a user agent might wish to coalesce timer
-       callbacks together, with no intermediate rendering updates.</p>
-      </div>
+      <p><i>Rendering opportunites</i>: If there are <span data-x="browsing context">browsing
+      contexts</span> <var>browsingContexts</var> that do not have a <span>rendering
+      opportunity</span>, then remove from <var>docs</var> all <code>Document</code> objects whose
+      <span data-x="concept-document-bc">browsing context</span> is in
+      <var>browsingContexts</var>.</p>
+
+      <p>A <span>browsing context</span> has a <dfn>rendering opportunity</dfn> if the user agent is
+      currently able to present the contents of the <span>browsing context</span> to the user,
+      accounting for hardware refresh rate constraints and user agent throttling for performance
+      reasons, but considering content presentable even if it's outside the viewport.</p>
+
+      <p><span>Browsing context</span> <span data-x="rendering opportunity">rendering
+      opportunities</span> are determined based on hardware constraints such as display refresh
+      rates and other factors such as page performance or whether the page is in the background.
+      Rendering opportunities typically occur at regular intervals.</p>
+
+      <p class="note">This specification does not mandate any particular model for selecting
+      rendering opportunities. But for example, if the browser is attempting to achieve a 60Hz
+      refresh rate, then rendering opportunities occur at a maximum of every 60th of a second (about
+      16.7ms). If the browser finds that a <span>browsing context</span> is not able to sustain this
+      rate, it might drop to a more sustainable 30 rendering opportunities per second for that
+      <span>browsing context</span>, rather than occasionally dropping frames. Similarly, if a
+      <span>browsing context</span> is not visible, the user agent might decide to drop that page to
+      a much slower 4 rendering opportunities per second, or even less.</p>
      </li>
 
+     <li><p><i>Unnecessary rendering</i>: If there are <span data-x="browsing context">browsing
+     contexts</span> <var>browsingContexts</var> for which the user agent believes updating the
+     rendering would have no visible effect and which possess no <code>Document</code> objects with
+     a non-empty <span>list of animation frame callbacks</span>, then remove from <var>docs</var>
+     all <code>Document</code> objects whose <span data-x="concept-document-bc">browsing
+     context</span> is in <var>browsingContexts</var>. Invoke the <span>mark paint timing</span>
+     algorithm for each <code>Document</code> object removed.</p></li>
+
      <li>
-      <p>If there are a <span data-x="nested browsing context">nested browsing contexts</span>
-      <var>B</var> that the user agent believes would not benefit from having their rendering
-      updated at this time, then remove from <var>docs</var> all <code>Document</code> objects whose
-      <span data-x="concept-document-bc">browsing context</span> is in <var>B</var>.</p>
-
-      <p class="note">As with <span data-x="top-level browsing context">top-level browsing
-      contexts</span>, a variety of factors can influence whether it is profitable for a browser to
-      update the rendering of <span data-x="nested browsing context">nested browsing
-      contexts</span>. For example, a user agent might wish to spend less resources rendering
-      third-party content, especially if it is not currently visible to the user or if resources are
-      constrained. In such cases, the browser could decide to update the rendering for such content
-      infrequently or never.</p>
+      <p>If there are <span data-x="browsing context">browsing contexts</span>
+      <var>browsingContexts</var> for which the user agent believes it's preferrable to skip
+      updating the rendering for other reasons, then remove from <var>docs</var> all
+      <code>Document</code> objects whose <span data-x="concept-document-bc">browsing context</span>
+      is in <var>browsingContexts</var>.</p>
+
+      <div class="note">
+       <p>The step labeled <i>Rendering opportunities</i> prevents the user agent from updating the
+       rendering when it is unable to present new content to the user (there's no <span>rendering
+       opportunity</span>).</p>
+
+       <p>The step labeled <i>Unnecessary rendering</i> prevents the user agent from updating the
+       rendering when there's no new content to draw.</p>
+
+       <p>This step enables the user agent to prevent the steps below from running for other
+       reasons, for example, to ensure certain <span data-x="concept-task">tasks</span> are executed
+       immediately after each other, with only <span data-x="perform a microtask
+       checkpoint">microtask checkpoints</span> interleaved (and without, e.g., <span data-x="run
+       the animation frame callbacks">animation frame callbacks</span> interleaved). Concretely, a
+       user agent might wish to coalesce timer callbacks together, with no intermediate rendering
+       updates.</p>
+      </div>
      </li>
 
      <li><p>For each <span>fully active</span> <code>Document</code> in <var>docs</var>, <span>run
@@ -89203,6 +89227,9 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
      the update intersection observations steps</span> for that <code>Document</code>, passing in
      <var>now</var> as the timestamp. <ref spec="INTERSECTIONOBSERVER"></p></li>
 
+     <li><p>Invoke the <span>mark paint timing</span> algorithm for each <code>Document</code>
+     object in <var>docs</var>.</p></li>
+
      <li><p>For each <span>fully active</span> <code>Document</code> in <var>docs</var>, update the
      rendering or user interface of that <code>Document</code> and its <span
      data-x="concept-document-bc">browsing context</span> to reflect the current state.</p></li>
@@ -121911,6 +121938,7 @@ INSERT INTERFACES HERE
   Thomas Koetter,
   Thomas O'Connor,
   Tim Altman,
+  Tim Dresser,
   Tim Johansson,
   Tim Perry,
   Tim van der Lippe,