Tweak "is unloading" in navigation

This replaces vague text about "if the prompt to unload algorithm is being run" and the like for "unload" with an explicit check of the "unload counter". The "unload counter" is a rename of the "ignore-opens-during-unload counter", which was already used for similar explicit checks in document.open().

This results in the same check being done everywhere, which appears to match at least some implementations better. Previously, for example, "traverse the history by a delta" did not check for beforeunload, only unload.

source

@@ -84029,8 +84029,8 @@ interface <dfn>History</dfn> {
    <li><p>If <var>source browsing context</var> is not <span>allowed to navigate</span>
    <var>specified browsing context</var>, then return.</p></li>
 
-   <li><p>If the <var>specified browsing context</var>'s <span>active document</span>'s
-   <span>unload a document</span> algorithm is currently running, return.</p></li>
+   <li><p>If the <var>specified browsing context</var>'s <span>active document</span>'s <span>unload
+   counter</span> is greater than 0, then return.</p></li>
 
    <li>
     <p><span>Queue a global task</span> on the <span>history traversal task source</span> given
@@ -85450,20 +85450,13 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
     </ol>
    </li>
 
-   <li><p>If there is a preexisting attempt to navigate <var>browsingContext</var>, and the
-   <span>source browsing context</span> is the same as <var>browsingContext</var>, and that attempt
-   is currently running the <span>unload a document</span> algorithm, then return without
-   affecting the preexisting attempt to navigate <var>browsingContext</var>.</p></li>
+   <li><p>If <var>browsingContext</var>'s <span>active document</span>'s <span>unload counter</span>
+   is greater than 0, then return.</p></li>
    <!--
      this stops pages from hijacking the back/forward button, among other things
      https://www.hixie.ch/tests/adhoc/html/navigation/unload/
      https://github.com/whatwg/html/issues/1213
-   -->
-
-   <li><p>If the <span>prompt to unload</span> algorithm is being run for the <span>active
-   document</span> of <var>browsingContext</var>, then return without affecting the
-   <span>prompt to unload</span> algorithm.</p></li>
-   <!-- https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=1946 to 1955 -->
+     https://software.hixie.ch/utilities/js/live-dom-viewer/?saved=1946 to 1955 -->
 
    <li id="navigate-fragid-step">
     <p>If <var>historyHandling</var> is not "<code data-x="hh-reload">reload</code>",
@@ -87677,15 +87670,18 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
   <p><span data-x="event loop">Event loops</span> have a <dfn>termination nesting level</dfn>
   counter, which must initially be 0.</p>
 
+  <p id="ignore-opens-during-unload-counter"><code>Document</code> objects have an <dfn>unload
+  counter</dfn>, which is used to ignore certain operations while the below algorithms run.
+  Initially, the counter must be set to zero.</p>
+
   <p>To <dfn id="prompt-to-unload-a-document">prompt to unload</dfn>, given a <code>Document</code>
   object <var>document</var> and optionally a <var>recursiveFlag</var>, run these steps:</p>
 
   <ol>
    <li><p>Increase the <span>event loop</span>'s <span>termination nesting level</span> by
    1.</p></li>
 
-   <li><p>Increase the <var>document</var>'s <span>ignore-opens-during-unload counter</span> by
-   1.</p></li>
+   <li><p>Increase the <var>document</var>'s <span>unload counter</span> by 1.</p></li>
 
    <li><p>Let <var>event</var> be the result of <span>creating an event</span> using
    <code>BeforeUnloadEvent</code>.</p></li>
@@ -87753,8 +87749,7 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
     </ol>
    </li>
 
-   <li><p>Decrease the <var>document</var>'s <span>ignore-opens-during-unload counter</span> by
-   1.</p></li>
+   <li><p>Decrease the <var>document</var>'s <span>unload counter</span> by 1.</p></li>
   </ol>
 
   <p>To <dfn export data-x="unload a document" data-lt="unload a document">unload</dfn> a
@@ -87765,8 +87760,7 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
    <li><p>Increase the <span>event loop</span>'s <span>termination nesting level</span> by
    one.</p></li>
 
-   <li><p>Increase <var>document</var>'s <span>ignore-opens-during-unload counter</span> by
-   one.</p></li>
+   <li><p>Increase <var>document</var>'s <span>unload counter</span> by 1.</p></li>
 
    <li><p>If <var>document</var>'s <span>page showing</span> flag is false, then jump to the
    step labeled <i>unload event</i> below (i.e. skip firing the <code
@@ -87841,8 +87835,7 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
     </ol>
    </li>
 
-   <li><p>Decrease <var>document</var>'s <span>ignore-opens-during-unload counter</span> by
-   one.</p></li>
+   <li><p>Decrease <var>document</var>'s <span>unload counter</span> by 1.</p></li>
   </ol>
 
   <p>This specification defines the following <dfn export>unloading document cleanup steps</dfn>.
@@ -87965,8 +87958,8 @@ interface <dfn>BeforeUnloadEvent</dfn> : <span>Event</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>
+   <var>document</var>'s <span>unload counter</span> is 0, 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>
@@ -94000,11 +93993,6 @@ document.body.appendChild(frame)</code></pre>
 
   <div w-nodev>
 
-  <p><code>Document</code> objects have an <dfn>ignore-opens-during-unload counter</dfn>, which is
-  used to prevent scripts from invoking the <code data-x="dom-document-open">document.open()</code>
-  method (directly or indirectly) while the document is <span data-x="unload a document">being
-  unloaded</span>. Initially, the counter must be set to zero.</p>
-
   <p><code>Document</code> objects have an <dfn>active parser was aborted</dfn> boolean, which is
   used to prevent scripts from invoking the <code data-x="dom-document-open">document.open()</code>
   and <code data-x="dom-document-write">document.write()</code> methods (directly or indirectly)
@@ -94038,8 +94026,8 @@ document.body.appendChild(frame)</code></pre>
    </li>
 
    <li>
-    <p>Similarly, if <var>document</var>'s <span>ignore-opens-during-unload counter</span> is
-    greater than 0, then return <var>document</var>.</p>
+    <p>Similarly, if <var>document</var>'s <span>unload counter</span> is greater than 0, then
+    return <var>document</var>.</p>
 
     <p class="note">This basically causes <code data-x="dom-document-open">document.open()</code> to
     be ignored when it's called from a <code data-x="event-beforeunload">beforeunload</code>, <code
@@ -94278,9 +94266,9 @@ document.body.appendChild(frame)</code></pre>
     <p>If the <span>insertion point</span> is undefined, then:
 
     <ol>
-     <li><p>If <var>document</var>'s <span>ignore-opens-during-unload counter</span> is greater than
-     0 or <var>document</var>'s <span>ignore-destructive-writes counter</span> is greater than 0,
-     then return.</p></li>
+     <li><p>If <var>document</var>'s <span>unload counter</span> is greater than 0 or
+     <var>document</var>'s <span>ignore-destructive-writes counter</span> is greater than 0, then
+     return.</p></li>
 
      <li><p>Run the <span>document open steps</span> with <var>document</var>.</p></li>
     </ol>