document.open(): only abort when there is a navigation

This implements the "ideal 2" plan in whatwg#3975, which was found to be
compatible with the existing Chrome test suite while being reasonably
straightforward.

Closes whatwg#3651.
Fixes whatwg#3975.

Tests: web-platform-tests/wpt#10789

source

@@ -78530,11 +78530,8 @@ interface <dfn data-export="" data-dfn-type="interface">Window</dfn> : <span>Eve
   been <span data-x="a browsing context is discarded">discarded</span>, and false otherwise.</p>
 
   <p>The <dfn><code data-x="dom-window-stop">stop()</code></dfn> method on <code>Window</code>
-  objects should, if there is an existing attempt to <span>navigate</span> the <span>browsing
-  context</span> and that attempt is not currently running the <span>unload a document</span>
-  algorithm, cancel that <span data-x="navigate">navigation</span>; then, it must <span
-  data-x="abort a document">abort</span> the <span>active document</span> of the <span>browsing
-  context</span> of the <code>Window</code> object on which it was invoked.</p>
+  objects must <span>stop document loading</span> given this <code>Window</code> object's <span
+  data-x="concept-document-window">associated <code>Document</code></span>.</p>
 
   </div>
 
@@ -83776,6 +83773,22 @@ interface <dfn>BeforeUnloadEvent</dfn> : <span>Event</span> {
 
   <!-- I'd love to make this more precise, anyone have any suggestions on what it should say? -->
 
+  <p>To <dfn>stop document loading</dfn> given a <code>Document</code> object <var>document</var>,
+  run these steps:</p>
+
+  <ol>
+   <li><p>If <var>document</var> is not an <span>active document</span>, then return.</p></li>
+
+   <li><p>Let <var>browsingContext</var> be <var>document</var>'s <span
+   data-x="concept-document-bc">browsing context</span>.</p></li>
+
+   <li><p>If there is an existing attempt to <span>navigate</span> <var>browsingContext</var> and
+   that attempt is not currently running the <span>unload a document</span> algorithm, then cancel
+   that <span data-x="navigate">navigation</span>.</p></li>
+
+   <li><p><span data-x="abort a document">Abort</span> <var>document</var>.</p></li>
+  </ol>
+
   </div>
 
 
@@ -90987,8 +91000,18 @@ document.body.appendChild(frame)</code></pre>
     handler while the <code>Document</code> is being unloaded.</p>
    </li>
 
-   <li><p>If <var>document</var> is an <span>active document</span>, then <span data-x="abort a
-   document">abort</span> <var>document</var>.</p></li>
+   <li>
+    <p>If there is an existing attempt to <span>navigate</span> <var>document</var>'s <span
+    data-x="concept-document-bc">browsing context</span>, then <span>stop document loading</span>
+    given <var>document</var>.</p>
+
+    <p class="&#x0058;&#x0058;&#x0058;">Issue <a
+    href="https://github.com/whatwg/html/issues/3447">#3447</a> looks into the distinction between
+    an ongoing instance of the <span>navigate</span> algorithm versus tasks to <span>navigate</span>
+    that are still queued. For the purpose of implementing this step, both an ongoing instance of
+    the <span>navigate</span> algorithm and tasks queued to <span>navigate</span> should be counted
+    towards "an existing attempt to <span>navigate</span>," before that issue is resolved.</p>
+   </li>
 
    <li><p>For each <span>shadow-including inclusive descendant</span> <var>node</var> of
    <var>document</var>, <span>erase all event listeners and handlers</span> given