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 +908,16 @@ for historical reasons.
 <h3 id=interface-eventtarget>Interface {{EventTarget}}</h3>
 
 <pre class=idl>
+[Exposed=*]
+interface EventTargetInternals {
+  attribute EventTarget parent;
+}
+
+callback EventTargetCallback = undefined (EventTargetInternals internals);
+
 [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 = {});
@@ -939,6 +946,33 @@ 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.
+<!-- Intentionally not exported. -->
+
+<p>When a <dfn export for=Event id=concept-eventtarget-constructor>constructor</dfn> of the {{EventTarget}}
+interface, or of an interface that inherits from the {{EventTarget}} interface, is invoked, these steps
+must be run, given the arguments <var>eventTargetCallback</var>:
+
+<ol>
+ <li><p>Let <var>eventTarget</var> be the result of creating a new object using this interface.
+
+ <li><p>Initialize <var>eventTarget</var>'s {{EventTarget/event listener list}} attribute to the empty list.
+
+ <li><p>Initialize <var>eventTarget</var>'s {{EventTarget/parent}} attribute to null.
+
+ <li><p>Let <var>eventTargetInternals</var> a new {{EventTargetInternals}} instance.
+
+ <li><p>If <var>eventTargetCallback</var> is not null, then <a
+ for=/invoke-a-callback-function>invoke</a> <var>eventTargetCallback</var> with «
+  <var>eventTargetInternals</var> and with <var>eventTarget</var> aas the <a>callback this value</a>.
+
+ <li><p>Set <var>eventTarget</var>'s {{Eventtarget/attached internals}} attribute to
+ <var>eventTargetInternals</var>.
+
+ <li><p>Return <var>eventTarget</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:
 
@@ -956,8 +990,9 @@ when something has occurred.
 object, an <a>event listener</a> is a broader concept as can be seen above.
 
 <p>Each {{EventTarget}} object also has an associated <dfn export>get the parent</dfn> algorithm,
-which takes an <a>event</a> <var>event</var>, and returns an {{EventTarget}} object. Unless
-specified otherwise it returns null.
+which takes an <a>event</a> <var>event</var>, and returns an {{EventTarget}}. Unless otherwise
+specified otherwise it returns the associated <a for=EventTarget>attached internals</a><a
+for=EventTargetInternals">parent</a> attribute.
 
 <p class=note><a for=/>Nodes</a>, <a for=/>shadow roots</a>, and <a>documents</a>
 override the <a>get the parent</a> algorithm.
@@ -1296,6 +1331,8 @@ property of the event being dispatched.
   <ol>
    <li><p>Let <var>touchTargets</var> be a new <a for=/>list</a>.
 
+   <li><p>Let <var>parentTargets</var> be a new <a for=/>list</a>.
+
    <li><p><a for=list>For each</a> <var>touchTarget</var> of <var>event</var>'s
    <a for=Event>touch target list</a>, <a for=list>append</a> the result of <a>retargeting</a>
    <var>touchTarget</var> against <var>target</var> to <var>touchTargets</var>.
@@ -1322,6 +1359,11 @@ property of the event being dispatched.
     <p>While <var>parent</var> is non-null:</p>
 
     <ol>
+     <li>If <var>parentTargets</var> <a for=/>contains</a> <var>parent</var> then <a>throw</a> a
+     "{{HierarchyRequestError!!exception}}" {{DOMException}}.
+
+     <li>Append <var>parent</var> to <var>parentTargets</var>.
+
      <li>
       <p>If <var>slottable</var> is non-null: