Add initial scroll position hook to HTML spec

source

@@ -3909,6 +3909,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-overflow/#propdef-overflow">'overflow'</dfn> property and its <dfn data-x-href="https://drafts.csswg.org/css-overflow/#valdef-overflow-hidden">'hidden'</dfn> value</li>
       <li>The <dfn data-x-href="https://drafts.csswg.org/css-overflow/#propdef-text-overflow">'text-overflow'</dfn> property</li>
       <li>The term <dfn data-x-href="https://drafts.csswg.org/css-overflow/#scroll-container">scroll container</dfn>
+      <li>The term <dfn data-x-href="https://drafts.csswg.org/css-overflow/#initial-scroll-position">initial scroll position</dfn>
+      <li>The term <dfn data-x-href="https://drafts.csswg.org/css-overflow/#scrollable-overflow-rectangle">scrollable overflow rectangle</dfn>
     </ul>
 
     <p>The following terms and features are defined in <cite>CSS Positioned Layout</cite>:
@@ -4061,6 +4063,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x="event-scrollend" data-x-href="https://drafts.csswg.org/cssom-view/#eventdef-document-scrollend"><code>scrollend</code></dfn> event</li>
      <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#set-up-browsing-context-features">set up browsing context features</dfn></li>
      <li>The <dfn data-x="mouseevent-clientx" data-x-href="https://drafts.csswg.org/cssom-view/#dom-mouseevent-clientx">clientX</dfn> and <dfn data-x="mouseevent-clienty" data-x-href="https://drafts.csswg.org/cssom-view/#dom-mouseevent-clienty">clientY</dfn> extension attributes of the <span>MouseEvent</span> interface</li>
+     <li>The term <dfn data-x-href="https://drafts.csswg.org/cssom-view/#perform-a-scroll">perform a scroll</dfn></li>
+     <li>The term <dfn data-x-href="https://drafts.csswg.org/cssom-view/#scrolling-box">scrolling box</dfn></li>
     </ul>
 
     <p>The following features and terms are defined in <cite>CSS Syntax</cite>:
@@ -104568,6 +104572,9 @@ location.href = '#foo';</code></pre>
     <p>If <var>documentIsNew</var> is true, then:
 
     <ol>
+     <li><p><span>Update the initial scroll positions for scroll containers</span> in
+     <var>document</var>.</p></li>
+
      <li><p><span>Try to scroll to the fragment</span> for <var>document</var>.</p></li>
 
      <li><p>At this point <dfn>scripts may run for the newly-created document</dfn>
@@ -104700,6 +104707,8 @@ location.href = '#foo';</code></pre>
    </li>
   </ol>
 
+  <hr>
+
   <p>To <dfn>try to scroll to the fragment</dfn> for a <code>Document</code> <var>document</var>,
   perform the following steps <span>in parallel</span>:</p>
 
@@ -104725,6 +104734,65 @@ location.href = '#foo';</code></pre>
    </li>
   </ol>
 
+  <p>To <dfn>update the initial scroll positions for scroll containers</dfn> in a
+  <code>Document</code> <var>document</var>:</p>
+
+  <ol>
+   <li><p>Let <var>scrollers</var> be the <span>list</span> obtained by performing a
+   <span>tree order</span> traversal of <var>document</var>'s <span>flat tree</span>, and filtering
+   to keep only the elements that establish a <span>scrolling box</span>.</p></li>
+
+   <li>
+    <p><span data-x="list iterate">For each</span> <var>scroller</var> of <var>scrollers</var>:</p>
+
+    <ol>
+     <li><p><span>Update the initial scroll position</span> for <var>scroller</var>.</p></li>
+    </ol>
+   </li>
+  </ol>
+
+  <p>To <dfn>update the initial scroll position</dfn> for an element <var>scroller</var>, perform
+  the following steps <span>in parallel</span>:</p>
+
+  <ol>
+   <li><p>Wait for an <span>implementation-defined</span> amount of time. (This is intended to allow
+   the user agent to optimize the user experience in the face of performance concerns.)</p></li>
+
+   <li>
+    <p><span>Queue a global task</span> on the <span>navigation and traversal task source</span>
+    given <var>document</var>'s <span>relevant global object</span> to run these steps:</p>
+
+    <ol>
+     <li><p>If the user agent has reason to believe the user is no longer interested in scrolling to
+     the <span data-x="initial scroll position">initial scroll position</span> of the
+     <span>scrollable overflow rectangle</span> of <var>scroller</var>'s <span>scrolling box</span>,
+     then abort these steps.</p></li>
+
+     <li><p>Let <var>position</var> be the <span data-x="initial scroll position">initial scroll
+     position</span> of the <span>scrollable overflow rectangle</span> of <var>scroller</var>'s
+     <span>scrolling box</span>.</p></li>
+
+     <li><p><span data-x="perform a scroll">Perform a scroll</span> of <var>scroller</var>'s <span
+     data-x="scrolling box">scrolling box</span> to <var>position</var>, with <var>scroller</var> as
+     the associated element.</p></li>
+
+     <li><p><span>Update the initial scroll position</span> for <var>scroller</var>.</p></li>
+    </ol>
+   </li>
+  </ol>
+
+  <p class="note">Although <span>try to scroll to the fragment</span> and <span>update the initial
+  scroll positions for scroll containers</span> take place at the same point, and both use a polling
+  system to prediodically re-attempt the scrolling, their goals are different. <span>Try to scroll
+  to the fragment</span> polls repeatedly until the <span>indicated part</span> is found since it
+  might take a while for the element to show up during the document load. <span>Update the initial
+  scroll positions for scroll containers</span> does not have such a finish condition; it can run
+  indefinitely, as long as the user does not indicate disinterest. This is necessary to achieve the
+  desired behavior of keeping a <span>scroll container</span> anchored to the position indicated by
+  CSS properties that determine the <span>initial scroll position</span>.</p>
+
+  <hr>
+
   <p>To <dfn export>make document unsalvageable</dfn>, given a <code>Document</code>
   <var>document</var> and a string <var>reason</var>:</p>
 
@@ -146275,6 +146343,7 @@ INSERT INTERFACES HERE
   Dave Singer,
   Dave Tapuska,
   Dave Townsend<!-- Mossop on moz irc -->,
+  David Awogbemila,
   David Baron,
   David Bloom,
   David Bokan,