Align spec with implementation experience

* The constructor needs to _consume_ transient user activation, not just check it.
* We need a reentrancy flag for the cancel event.
* We need to not fire close if destroy() was called during the cancel event handler.

spec.bs

@@ -148,13 +148,20 @@ A <dfn export>close watcher</dfn> is a [=struct=] with the following [=struct/it
 </div>
 
 <div algorithm>
-  We <dfn lt="can create a developer-controlled close watcher|cannot create a developer-controlled close watcher">can create a developer-controlled close watcher</dfn> for a {{Document}} |document| if the following steps return true:
+  To <dfn lt="check if we can create a developer-controlled close watcher">check if we can create a developer-controlled close watcher</dfn> for a {{Window}} |window|:
 
+  1. Let |document| be |window|'s [=associated Document=].
   1. If |document| is not [=Document/fully active=], then return false.
-  1. If |document|'s [=relevant global object=] has [=transient activation=], then return true.
+  1. Let |needsUserActivation| be false.
   1. [=list/For each=] |closeWatcher| in |document|'s [=close watcher stack=]:
-    1. If |closeWatcher|'s [=close watcher/is still valid=] steps return true, and |closeWatcher|'s [=close watcher/blocks further developer-controlled close watchers=] is true, then return false.
-  1. Return true.
+    1. If |closeWatcher|'s [=close watcher/is still valid=] steps return true, and |closeWatcher|'s [=close watcher/blocks further developer-controlled close watchers=] is true, then set |needsUserActivation| to true and [=iteration/break=].
+  1. Let |canCreate| be false.
+  1. If |needsUserActivation| is false, then set |canCreate| to true.
+  1. Otherwise, if |window| has [=transient activation=], then:
+    1. [=Consume user activation=] given |window|.
+    1. Set |canCreate| to true.
+  1. If |canCreate| is true, then set |window|'s [=timestamp of last activation used for close watchers=] to |window|'s <a spec="HTML">last activation timestamp</a>.
+  1. Return |canCreate|.
 </div>
 
 <h3 id="close-watcher-api">Close watcher API</h3>
@@ -195,19 +202,19 @@ interface CloseWatcher : EventTarget {
   </dd>
 </dl>
 
-Each {{CloseWatcher}} has an <dfn for="CloseWatcher">is active</dfn>, which is a boolean.
+Each {{CloseWatcher}} has an <dfn for="CloseWatcher">is active</dfn>, which is a boolean, and an <dfn for="CloseWatcher">firing cancel event</dfn>, which is a boolean.
 
 <div algorithm>
   The <dfn constructor for="CloseWatcher" lt="CloseWatcher()">new CloseWatcher()</dfn> constructor steps are:
 
   1. If [=this=]'s [=relevant global object=]'s [=associated Document=] is not [=Document/fully active=], then throw an "{{InvalidStateError}}" {{DOMException}}.
-  1. If we [=cannot create a developer-controlled close watcher=] for [=this=]'s [=relevant global object=]'s [=associated document=], then throw a "{{NotAllowedError}}" {{DOMException}}.
+  1. If the result of [=checking if we can create a developer-controlled close watcher=] for [=this=]'s [=relevant global object=] is false, then throw a "{{NotAllowedError}}" {{DOMException}}.
   1. Set [=this=]'s [=CloseWatcher/is active=] to true.
+  1. Set [=this=]'s [=CloseWatcher/firing cancel event=] to false.
   1. [=stack/Push=] a new [=close watcher=] on [=this=]'s [=relevant global object=]'s [=associated document=]'s [=close watcher stack=], with its [=struct/items=] set as follows:
     * [=close watcher/close action=] being to [=CloseWatcher/signal close=] on [=this=]
     * [=close watcher/is still valid=] steps being to return [=this=]'s [=CloseWatcher/is active=]
     * [=close watcher/blocks further developer-controlled close watchers=] being true
-  1. Set [=this=]'s [=relevant global object=]'s [=timestamp of last activation used for close watchers=] to [=this=]'s [=relevant global object=]'s <a spec="HTML">last activation timestamp</a>.
 </div>
 
 <p algorithm>
@@ -224,12 +231,15 @@ Objects implementing the {{CloseWatcher}} interface must support the <dfn attrib
   To <dfn for="CloseWatcher">signal close</dfn> on a {{CloseWatcher}} |closeWatcher|:
 
   1. If |closeWatcher|'s [=CloseWatcher/is active=] is false, then return.
+  1. If |closeWatcher|'s [=CloseWatcher/firing cancel event=] is true, then return.
   1. Let |window| be |closeWatcher|'s [=relevant global object=].
   1. If |window|'s [=associated Document=] is [=Document/fully active=], and |window|'s [=timestamp of last activation used for close watchers=] does not equal |window|'s <a spec="HTML">last activation timestamp</a>, then:
-    1. Let |shouldContinue| be the result of [=firing an event=] named {{CloseWatcher/cancel}} at |closeWatcher|, with the {{Event/cancelable}} attribute initialized to true.
     1. Set |window|'s [=timestamp of last activation used for close watchers=] to |window|'s <a spec="HTML">last activation timestamp</a>.
+    1. Set |closeWatcher|'s [=CloseWatcher/firing cancel event=] to true.
+    1. Let |shouldContinue| be the result of [=firing an event=] named {{CloseWatcher/cancel}} at |closeWatcher|, with the {{Event/cancelable}} attribute initialized to true.
+    1. Set |closeWatcher|'s [=CloseWatcher/firing cancel event=] to false.
     1. If |shouldContinue| is false, then return.
-  1. If |window|'s [=associated Document=] is [=Document/fully active=], then [=fire an event=] named {{CloseWatcher/close}} at |closeWatcher|.
+  1. If |closeWatcher|'s [=CloseWatcher/is active=] is true, and |window|'s [=associated Document=] is [=Document/fully active=], then [=fire an event=] named {{CloseWatcher/close}} at |closeWatcher|.
   1. Set |closeWatcher|'s [=CloseWatcher/is active=] to false.
 </div>
 
@@ -248,12 +258,12 @@ Update <cite>HTML</cite>'s <a href="https://html.spec.whatwg.org/multipage/inter
 <div algorithm="showModal patch">
   In the {{HTMLDialogElement/showModal()}} steps, after adding |subject| to the [=top layer=], append the following step:
 
-  1. If we [=can create a developer-controlled close watcher=] given |subject|'s [=Node/node document=], then [=stack/push=] a new [=close watcher=] on |subject|'s [=Node/node document=]'s [=close watcher stack=], with its [=struct/items=] set as follows:
+  1. If the result of [=checking if we can create a developer-controlled close watcher=] given |subject|'s [=relevant global object=] is true, then [=stack/push=] a new [=close watcher=] on |subject|'s [=Node/node document=]'s [=close watcher stack=], with its [=struct/items=] set as follows:
     * [=close watcher/close action=] being to [=cancel the dialog=] |subject|
     * [=close watcher/is still valid=] steps being to return true if |subject|'s [=Node/node document=] is <a spec="HTML" lt="blocked by a modal dialog">blocked by the modal dialog</a> |subject|, and return false otherwise
     * [=close watcher/blocks further developer-controlled close watchers=] being true
 
-    <p class="note">If we [=cannot create a developer-controlled close watcher=], then this modal dialog will not respond to [=close signals=]. The {{HTMLDialogElement/showModal()}} method proceeds without any exception or other indication of this, although the browser could [=report a warning to the console=].
+    <p class="note">If we cannot create a developer-controlled close watcher, then this modal dialog will not respond to [=close signals=]. The {{HTMLDialogElement/showModal()}} method proceeds without any exception or other indication of this, although the browser could [=report a warning to the console=].
 </div>
 
 Replace the "Canceling dialogs" section entirely with the following definition. (The previous prose about providing a user interface to cancel such dialogs, and the task-queuing, is now handled by the infrastructure in [[#close-signals]].)