Gate beforeunload dialogs behind sticky activation

Closes #6139.

Author: domenic

source

@@ -86177,20 +86177,28 @@ dictionary <dfn>PageTransitionEventInit</dfn> : <span>EventInit</span> {
    1.</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
-    data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute of the <var>event</var>
-    object is not the empty string, or if the event was canceled, then the user agent may ask the
-    user to confirm that they wish to unload the document.</p>
+    <p>If all of the following are true:</p>
+
+    <ul class="brief">
+     <li><p><var>document</var>'s <span>active sandboxing flag set</span> does not have its
+     <span>sandboxed modals flag</span> set</p></li>
+
+     <li><p><var>document</var>'s <span>relevant global object</span> has <span>sticky
+     activation</span></p></li>
+
+     <li><p><var>event</var>'s <span>canceled flag</span> is set, or the <code
+     data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute of <var>event</var> is
+     not the empty string</p></li>
+    </ul>
+
+    <p>then the user agent may ask the user to confirm that they wish to unload the document.</p>
 
     <p class="note">The message shown to the user is not customizable, but instead determined by
     the user agent. In particular, the actual value of the <code
     data-x="dom-BeforeUnloadEvent-returnValue">returnValue</code> attribute is ignored.</p>
 
     <p>The user agent is encouraged to avoid asking the user for confirmation if it judges that
-    doing so would be annoying, deceptive, or pointless. A simple heuristic might be that if the
-    user has not interacted with the document, the user agent would not ask for confirmation before
-    unloading it.</p>
+    doing so would be annoying, deceptive, or pointless.</p>
 
     <p>If the user agent asks the user for confirmation, it must <span>pause</span> while waiting
     for the user's response.</p>