Allow for customisation of the "get the parent" algorithm

This modifies the EventTarget IDL to allow assignment of a parent
EventTarget to an EventTarget instance, while also modifying the "get
the parent" algorithm to default to returning that instance.

It also modifies the Event Dispatch algorithm to ensure that custom
parent chains cannot cause loops.

dom.bs

@@ -908,9 +908,14 @@ for historical reasons.
 <h3 id=interface-eventtarget>Interface {{EventTarget}}</h3>
 
 <pre class=idl>
+[Exposed=*]
+interface EventTargetInternals {
+  attribute EventTarget parent;
+};
+
 [Exposed=*]
 interface EventTarget {
-  constructor();
+  constructor(optional EventTargetCallback cb);
 
   undefined addEventListener(DOMString type, EventListener? callback, optional (AddEventListenerOptions or boolean) options = {});
   undefined removeEventListener(DOMString type, EventListener? callback, optional (EventListenerOptions or boolean) options = {});
@@ -930,6 +935,8 @@ dictionary AddEventListenerOptions : EventListenerOptions {
   boolean once = false;
   AbortSignal signal;
 };
+
+callback EventTargetCallback = undefined (EventTargetInternals internals);
 </pre>
 
 <p>An {{EventTarget}} object represents a target to which an <a>event</a> can be <a>dispatched</a>
@@ -939,6 +946,63 @@ when something has occurred.
 <a for=/>list</a> of zero or more <a>event listeners</a>). It is initially the empty list.
 <!-- Intentionally not exported. -->
 
+<p>Each {{EventTarget}} object has an associated <dfn for=EventTarget>attached internals</dfn> (null
+or an {{EventTargetInternals}} object), initially null.
+
+<p>Each {{EventTargetInternals}} object has an associated <dfn
+for=EventTargetInternals>eventTarget</dfn> (an {{EventTarget}} object).
+
+<p>The <dfn export for=Event id=concept-eventtarget-constructor>new EventTarget(cb)</dfn> constructor steps are:
+
+<ol>
+ <li><p>If <var>cb</var> is not null then:
+
+  <ol>
+   <li><p>Let <var>eventTargetInternals</var> a new {{EventTargetInternals}} instance.
+
+   <li><p>Set <var>eventTargetInternals</var>'s <a for=EventTargetInternals>eventTarget</a> to
+   <var>this</var>.
+
+   <li><a>Invoke</a> <var>cb</var> with « <var>eventTargetInternals</var> » and with <var>this</var>
+   as the <a>callback this value</a>.
+
+   <li><p>Set <var>this</var>'s <a for="EventTarget">attached internals</a> to
+   <var>eventTargetInternals</var>.
+  </ol>
+</ol>
+
+<p>To <dfn export for=Event id=concept-eventtargetinternals-set-parent>set the parent</dfn> given
+an {{EventTargetInternals}} <var>internals</var> and {{EventTarget}} <var>theParent</var>:
+
+<ol>
+  <li><p>Let <var>targets</var> be a new <a for=/>list</a>.
+
+   <li>Append <var>internals</var> <a for=EventTargetInternals>eventTarget</a> to <var>targets</var>.
+
+  <li><p>Let <var>parent</var> be <var>theParent</var>.
+
+  <li>
+   <p>While <var>parent</var> is non-null:</p>
+
+   <ol>
+    <li>If <var>targets</var> <a for=set>contains</a> <var>parent</var> then <a>throw</a> a
+    "{{HierarchyRequestError!!exception}}" {{DOMException}}.
+
+    <li>Append <var>parent</var> to <var>targets</var>.
+
+    <li>Let <var>parentInternals</var> be <var>parent</var>'s <a for=EventTarget>attached internals</a>.
+
+    <li>Set <var>parent</var> to <var>parentIntenrals</var>'s <a
+    for=EventTargetInternals>parent</a>.
+
+   </ol>
+
+  <li>Set <var>internal</var>'s <a for=EventTargetInternals>eventTarget</a> <a>get the parent</a>
+  algorithm to return <var>theParent</var>.
+
+  <li>Set <var>internal</var>'s {{EventTargetInternals/parent}} to <var>theParent</var>.
+</ol>
+
 <p>An <dfn export id=concept-event-listener>event listener</dfn> can be used to observe a specific
 <a>event</a> and consists of:
 
@@ -1322,6 +1386,9 @@ property of the event being dispatched.
     <p>While <var>parent</var> is non-null:</p>
 
     <ol>
+     <li>If the <var>event</var>'s <a for=Event>path</a> <a for=set>contains</a> <var>parent</var>
+     then <a>throw</a> a "{{HierarchyRequestError!!exception}}" {{DOMException}}.
+
      <li>
       <p>If <var>slottable</var> is non-null: