Change beforeunload, unload, and bfcache interaction

Per #5748 (comment),
removes the requirement that beforeunload event listeners being fired
cause a document to become unsalveagable (i.e., ineligible for bfcache).
Instead, unsalveagability is left more explicitly up to the user agent.

Additionally, changes navigation to no longer dispatch unload events for
documents which are kept alive in the session history (bfcache).

Closes #5748.

source

@@ -85167,8 +85167,8 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
     <p>Things that can cause the page to be unsalvageable include:</p>
 
     <ul class="brief">
-     <li>Listening for <code data-x="event-beforeunload">beforeunload</code> events
-     <li>Listening for <code data-x="event-unload">unload</code> events
+     <li>The user agent decided to not keep the <code>Document</code> alive in a <span>session
+     history entry</span> after <span data-x="unload a document">unload</span>
      <li>Having <code>iframe</code>s that are not salvageable
      <li>Active <code>WebSocket</code> objects
      <li><span data-x="abort a document">Aborting a <code>Document</code></span>
@@ -85220,12 +85220,12 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
 
   <p>A <code>Document</code> has a <dfn
   data-x="concept-document-salvageable"><i>salvageable</i></dfn> state, which must initially be
-  true, a <dfn>fired unload</dfn> flag, which must initially be false, and a <dfn>page showing</dfn>
-  flag, which must initially be false. The <span>page showing</span> flag is used to ensure that
-  scripts receive <code data-x="event-pageshow">pageshow</code> and <code
-  data-x="event-pagehide">pagehide</code> events in a consistent manner (e.g. that they never
-  receive two <code data-x="event-pagehide">pagehide</code> events in a row without an intervening
-  <code data-x="event-pageshow">pageshow</code>, or vice versa).</p>
+  true, and a <dfn>page showing</dfn> flag, which must initially be false. The <span>page
+  showing</span> flag is used to ensure that scripts receive <code
+  data-x="event-pageshow">pageshow</code> and <code data-x="event-pagehide">pagehide</code> events
+  in a consistent manner (e.g. that they never receive two <code
+  data-x="event-pagehide">pagehide</code> events in a row without an intervening <code
+  data-x="event-pageshow">pageshow</code>, or vice versa).</p>
 
   <p><span data-x="event loop">Event loops</span> have a <dfn>termination nesting level</dfn>
   counter, which must initially be 0.</p>
@@ -85253,10 +85253,6 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
    <li><p>Decrease the <span>event loop</span>'s <span>termination nesting level</span> by
    1.</p></li>
 
-   <li><p>If any event listeners were triggered by the earlier <i>dispatch</i> step, then set
-   <var>document</var>'s <i data-x="concept-document-salvageable">salvageable</i> state to
-   false.</p></li>
-
    <li>
     <p>If <var>document</var>'s <span>active sandboxing flag set</span> does not have its
     <span>sandboxed modals flag</span> set, and the <code
@@ -85292,15 +85288,11 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
 
       <ol>
        <li><p><span>Prompt to unload</span> <var>browsingContext</var>'s <span>active
-       document</span> with the <var>recursiveFlag</var> set. If the user <span>refused to allow
-       the document to be unloaded</span>, then the user implicitly also <span data-x="refused to
-       allow the document to be unloaded">refused to allow <var>document</var> to be
-       unloaded</span>; <span>break</span>.</p></li>
-
-       <li><p>If the <i data-x="concept-document-salvageable">salvageable</i> state of
-       <var>browsingContext</var>'s <span>active document</span> is false, then set the <i
-       data-x="concept-document-salvageable">salvageable</i> state of <var>document</var> to
-       false.</p></li>
+       document</span> with the <var>recursiveFlag</var> set.</p></li>
+
+       <li><p>If the user <span>refused to allow the document to be unloaded</span>, then the user
+       implicitly also <span data-x="refused to allow the document to be unloaded">refused to allow
+       <var>document</var> to be unloaded</span>; <span>break</span>.</p></li>
       </ol>
      </li>
     </ol>
@@ -85327,6 +85319,11 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
 
    <li><p>Set <var>document</var>'s <span>page showing</span> flag to false.</p></li>
 
+   <li><p>If the user agent does not intend to keep <var>document</var> alive
+   in a <span>session history entry</span> (such that it can be reused later on <span
+   data-x="traverse the history">history traversal</span>), set <var>document</var>'s
+   <i data-x="concept-document-salvageable">salvageable</i> state to false.</p></li>
+
    <li><p><span data-x="concept-event-fire">Fire an event</span> named <code
    data-x="event-pagehide">pagehide</code> at <var>document</var>'s <span>relevant global
    object</span>, using <code>PageTransitionEvent</code>, with the <code
@@ -85342,18 +85339,15 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
     <ref spec=PAGEVIS></p>
    </li>
 
-   <li><p><i>Unload event</i>: If <var>document</var>'s <span>fired unload</span> flag is
-   false, then <span data-x="concept-event-fire">fire an event</span> named <code
-   data-x="event-unload">unload</code> at <var>document</var>'s <span>relevant global object</span>,
-   with <var>legacy target override flag</var> set.</p></li>
+   <li><p><i>Unload event</i>: If <var>document</var>'s <i
+   data-x="concept-document-salvageable">salvageable</i> state is false, then <span
+   data-x="concept-event-fire">fire an event</span> named <code data-x="event-unload">unload</code>
+   at <var>document</var>'s <span>relevant global object</span>, with <var>legacy target override
+   flag</var> set.</p></li>
 
    <li><p>Decrease the <span>event loop</span>'s <span>termination nesting level</span> by
    one.</p></li>
 
-   <li><p>If any event listeners were triggered by the earlier <i>unload event</i> step, then set
-   <var>document</var>'s <i data-x="concept-document-salvageable">salvageable</i> state
-   to false and set <var>document</var>'s <span>fired unload</span> flag to true.</p></li>
-
    <li><p>Run any <span>unloading document cleanup steps</span> for <var>document</var> that are
    defined by this specification and <span>other applicable specifications</span>.</p></li>