Fix event handler processing algorithm to match implementations

* Remove the special cancelation behavior for onmouseover. This was only
  ever implemented by Gecko. Fixes #423 in part.
* Update the special cancelation and argument behavior for onerror to a
  reasonable set of intersection semantics among current
  implementations. This fixes #2296, and fixes the rest of #423. See
  especially
  #2296 (comment).
* Editorial: links directly to Web IDL's definition of "invoke" instead
  of indirecting.
* Editorial: clarifies that BeforeUnloadEvents always have type
  "beforeunload", instead of making that part of the conditional.

Author: domenic

source

@@ -88676,19 +88676,30 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
    <li><p>If <var>callback</var> is null, then abort these steps.</p></li>
 
+   <li>
+    <p>Let <var>special error event handling</var> be true if <var>E</var> is an
+    <code>ErrorEvent</code> object and <var>E</var>'s <code
+    data-x="dom-Event-currentTarget">currentTarget</code> implements the
+    <code>WindowOrWorkerGlobalScope</code> mixin. Otherwise, let <var>special error event
+    handling</var> be false.</p>
+
+    <p class="note">In this case, <var>E</var>'s <code data-x="dom-Event-type">type</code> will
+    always be <code data-x="event-error">error</code>, since the only <span data-x="event
+    handlers">event handler</span> is called with <code>ErrorEvent</code> instances is <code
+    data-x="handler-onerror">onerror</code>.</p>
+   </li>
+
    <li>
 
     <p>Process the <code>Event</code> object <var>E</var> as follows:</p>
 
     <dl class="switch">
 
-     <dt>If <var>E</var> is an <code>ErrorEvent</code> object and the <span data-x="event
-     handler IDL attributes" data-export="">event handler IDL attribute</span>'s type is
-     <code>OnErrorEventHandler</code></dt>
+     <dt>If <var>special error event handling</var> is true</dt>
 
      <dd>
 
-      <p><span data-x="concept-invoke-event-handler">Invoke</span> <var>callback</var> with five
+      <p><span data-x="es-invoking-callback-functions">Invoke</span> <var>callback</var> with five
       arguments, the first one having the value of <var>E</var>'s <code
       data-x="dom-ErrorEvent-message">message</code> attribute, the second having the value of
       <var>E</var>'s <code data-x="dom-ErrorEvent-filename">filename</code> attribute, the third
@@ -88706,17 +88717,14 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
      <dd>
 
-      <p><span data-x="concept-invoke-event-handler">Invoke</span> <var>callback</var>
+      <p><span data-x="es-invoking-callback-functions">Invoke</span> <var>callback</var>
       with one argument, the value of which is the <code>Event</code> object <var>E</var>,
       with the <i data-x="dfn-callback-this-value">callback this value</i> set to <var>E</var>'s <code data-x="dom-Event-currentTarget">currentTarget</code>. Let <var>return value</var> be the callback's return value. <ref spec=WEBIDL></p>
 
      </dd>
 
     </dl>
 
-    <p>In this step, <dfn data-x="concept-invoke-event-handler">invoke</dfn> means to <span
-    data-x="es-invoking-callback-functions">invoke the Web IDL callback function</span>.</p>
-
     <p>If an exception gets thrown by the callback, end these steps and allow the exception to
     propagate. (It will propagate to the <span data-x="concept-event-dispatch">DOM event dispatch
     logic</span>, which will then <span>report the exception</span>.)</p>
@@ -88729,19 +88737,14 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
     <dl class="switch">
 
-     <dt>If <var>E</var>'s <code data-x="dom-event-type">type</code> is <code
-     data-x="event-mouseover">mouseover</code></dt>
-     <dt>If <var>E</var>'s <code data-x="dom-event-type">type</code> is <code
-     data-x="event-error">error</code> and <var>E</var> is an <code>ErrorEvent</code> object</dt>
-     <dd><p>If <var>return value</var> is true, then set <var>E</var>'s <span>canceled
-     flag</span>.</p></dd>
-
-     <dt>If <var>E</var>'s <code data-x="dom-event-type">type</code> is <code
-     data-x="event-beforeunload">beforeunload</code> and <var>E</var> is a
-     <code>BeforeUnloadEvent</code> object</dt>
+     <dt>If <var>E</var> is a <code>BeforeUnloadEvent</code> object</dt>
      <dd>
-      <p class="note">The <span data-x="event handler IDL attributes">event handler IDL
-      attribute</span>'s type is <code>OnBeforeUnloadEventHandler</code>, and the <var>return
+      <p class="note">This can only occur when <var>E</var>'s <code
+      data-x="dom-Event-type">type</code> is <code data-x="event-beforeunload">beforeunload</code>
+      and when the <span data-x="event handler IDL attributes">event handler IDL attribute</span>'s
+      type is <code>OnBeforeUnloadEventHandler</code>, since the only <span data-x="event
+      handlers">event handler</span> that is called with <code>BeforeUnloadEvent</code> instances is
+      <code data-x="handler-window-onbeforeunload">onbeforeunload</code>. In this case, <var>return
       value</var> will therefore have been coerced into either the value null or a <code
       data-x="idl-DOMString">DOMString</code>.</p>
 
@@ -88757,12 +88760,14 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
       </ol>
      </dd>
 
+     <dt>If <var>special error event handling</var> is true</dt>
+     <dd><p>If <var>return value</var> is true, then set <var>E</var>'s <span>canceled
+     flag</span>.</p></dd>
+
      <dt>Otherwise</dt>
      <dd><p>If <var>return value</var> is false, then set <var>E</var>'s <span>canceled
      flag</span>.</p></dd>
 
-     <!-- IE actually uncancels the event if the function returns true -->
-
     </dl>
 
    </li>
@@ -88795,11 +88800,20 @@ typedef <span>EventHandlerNonNull</span>? <dfn>EventHandler</dfn>;</pre>
 
   </div>
 
-  <p class="note">The return value of the function affects whether the event is canceled or not:
-  <span w-nodev>as described above,</span> if the return value is false, the event is canceled
-  (except for <code data-x="event-mouseover">mouseover</code> events, where the return value has to
-  be true to cancel the event). With <code data-x="event-beforeunload">beforeunload</code> events,
-  the value is instead used to determine whether or not to prompt about unloading the document.</p>
+  <div class="note">
+   <p>The return value of the function affects whether the event is canceled or not: <span
+   w-nodev>as described above, </span>if the return value is false, the event is canceled.</p>
+
+   <p>There are two exceptions in the platform, for historical reasons:</p>
+
+   <ul>
+    <li><p>The <code data-x="handler-onerror">onerror</code> handlers on global objects, where
+    returning <em>true</em> cancels the event</p></li>
+
+    <li><p>The <code data-x="handler-window-onbeforeunload">onbeforeunload</code> handler, where
+    returning any non-null and non-undefined value will cancel the event.</p></li>
+   </ul>
+  </div>
 
   <p>For historical reasons, the <code data-x="handler-onerror">onerror</code> handler has different
   arguments:</p>