Add Imperative Slot API

dom.bs

@@ -2195,6 +2195,10 @@ steps:</p>
  <li><p>If the <i>open flag</i> is set and <var>shadow</var>'s <a for=ShadowRoot>mode</a> is
  <em>not</em> "<code>open</code>", then return null.</p></li>
 
+ <li><p>If <var>shadow</var>'s <a for=ShadowRoot>slot assignment</a> is set to "<code>manual</code>",
+ return the <a>slot</a> in <var>shadow</var>'s <a for=tree>descendants</a> whose <a>manually assigned nodes</a>
+ includes <var>slottable</var>, if any, and null otherwise.</p></li>
+
  <li><p>Return the first <a>slot</a> in <a>tree order</a> in <var>shadow</var>'s
  <a for=tree>descendants</a> whose <a for=slot>name</a> is <var>slottable</var>'s
  <a for=slottable>name</a>, if any, and null otherwise.</p></li>
@@ -2450,7 +2454,8 @@ before a <var>child</var>, with an optional <i>suppress observers flag</i>, run
    <li><p>Otherwise, <a for=set>insert</a> <var>node</var> into <var>parent</var>'s
    <a for=tree>children</a> before <var>child</var>'s <a for=tree>index</a>.
 
-   <li><p>If <var>parent</var> is a <a for=Element>shadow host</a> and <var>node</var> is a
+   <li><p>If <var>parent</var> is a <a for=Element>shadow host</a> with its <a for=tree>root</a>'s
+   <a for=ShadowRoot>slot assignment</a> set to "<code>auto</code>" and <var>node</var> is a
    <a>slottable</a>, then <a>assign a slot</a> for <var>node</var>.
 
    <li><p>If <var>parent</var>'s <a for=tree>root</a> is a <a for=/>shadow root</a>, and
@@ -2667,8 +2672,16 @@ indicated in the <a for=/>remove</a> algorithm below.
 
  <li><p><a for=set>Remove</a> <var>node</var> from its <var>parent</var>'s <a for=tree>children</a>.
 
- <li><p>If <var>node</var> is <a for=slottable>assigned</a>, then run <a>assign slottables</a> for
- <var>node</var>'s <a>assigned slot</a>.
+ <li>
+  <p>If <var>node</var> is <a for=slottable>assigned</a>, then:
+
+  <ol>
+   <li><p><p>If <var>parent</var>'s <a for=tree>root</a> is a <a for=/>shadow root</a> with its
+   <a for=tree>root</a>'s <a for=ShadowRoot>slot assignment</a> set to "<code>auto</code>",
+   then <a for=set>remove</a> <var>node</var> from its <a>assigned slot</a>'s <a>manually assigned nodes</a>.
+
+   <li><p>Run <a>assign slottables</a> for <var>node</var>'s <a>assigned slot</a>.
+  </ol>
 
  <li><p>If <var>parent</var>'s <a for=tree>root</a> is a <a for=/>shadow root</a>, and
  <var>parent</var> is a <a>slot</a> whose <a for=slot>assigned nodes</a> is the empty list,
@@ -2678,6 +2691,11 @@ indicated in the <a for=/>remove</a> algorithm below.
   <p>If <var>node</var> has an <a>inclusive descendant</a> that is a <a>slot</a>, then:
 
   <ol>
+   <li><p>for each <a>slot</a> <var>slot</var> in <var>node</var>'s <a for=tree>inclusive descendants</a>,
+   if <var>slot</var>'s <a for=tree>root</a> is a <a for=/>shadow root</a> with its <a for=tree>root</a>'s
+   <a for=ShadowRoot>slot assignment</a> set to "<code>manual</code>", then set <var>slot</var>'s
+   <a>manually assigned nodes</a> to an empty list.
+
    <li><p>Run <a>assign slottables for a tree</a> with <var>parent</var>'s <a for=tree>root</a>.
 
    <li><p>Run <a>assign slottables for a tree</a> with <var>node</var>.
@@ -5711,11 +5729,13 @@ invoked, must return a new {{DocumentFragment}} <a>node</a> whose <a for=Node>no
 [Exposed=Window]
 interface ShadowRoot : DocumentFragment {
   readonly attribute ShadowRootMode mode;
+  readonly attribute SlotAssignmentMode slotAssignment;
   readonly attribute Element host;
   attribute EventHandler onslotchange;
 };
 
 enum ShadowRootMode { "open", "closed" };
+enum SlotAssignmentMode { "manual", "auto" };
 </pre>
 
 <p>{{ShadowRoot}} <a for=/>nodes</a> are simply known as
@@ -5731,6 +5751,9 @@ It is initially set to false.</p>
 <!-- If we ever change this, e.g., add a ShadowRoot object constructor, that would have serious
      consequences for innerHTML. -->
 
+<p><a for=/>Shadow roots</a> have an associated <dfn for=ShadowRoot>slot assignment</dfn>
+("<code>manual</code>" or "<code>auto</code>").</p>
+
 <p>A <a for=/>shadow root</a>'s <a>get the parent</a> algorithm, given an <var>event</var>, returns
 null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow root</a> is the
 <a for=tree>root</a> of <var>event</var>'s <a for=Event>path</a>'s first struct's
@@ -5748,6 +5771,9 @@ null if <var>event</var>'s <a>composed flag</a> is unset and <a for=/>shadow roo
 <dfn for=ShadowRoot export><code>onslotchange</code></dfn> <a>event handler</a>, whose
 <a>event handler event type</a> is {{HTMLSlotElement/slotchange}}.
 
+<p>The <dfn attribute for=ShadowRoot><code>slotAssignment</code></dfn> attribute's getter must
+return <a>this</a>'s <a for=ShadowRoot>slot assignment</a>.</p>
+
 <hr>
 
 <p>In <dfn export id=concept-shadow-including-tree-order>shadow-including tree order</dfn> is
@@ -5871,6 +5897,7 @@ interface Element : Node {
 dictionary ShadowRootInit {
   required ShadowRootMode mode;
   boolean delegatesFocus = false;
+  SlotAssignmentMode slotAssignment = "auto";
 };
 </pre>
 
@@ -6744,6 +6771,9 @@ invoked, must run these steps:
  <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>delegates focus</a> to <var>init</var>'s
  {{ShadowRootInit/delegatesFocus}}.
 
+ <li><p>Set <var>shadow</var>'s <a for=ShadowRoot>slot assignment</a> to <var>init</var>'s
+ {{ShadowRootInit/slotAssignment}}.
+
  <li><p>Set <a>this</a>'s <a for=Element>shadow root</a> to <var>shadow</var>.
 
  <li><p>Return <var>shadow</var>.
@@ -10187,7 +10217,8 @@ Xidorn Quan,
 Yehuda Katz,
 Yoav Weiss,
 Yoichi Osato,
-Yoshinori Sano, and
+Yoshinori Sano,
+Yu Han, <!-- yuzhe-han on GitHub --> and
 Zack Weinberg
 for being awesome!