title: Dialog "Are you sure you want to leave this page?" shows just once in Safari
body: <p><strong>Issue:</strong></p>
  
  <p>To show the dialog (<a href="http://prntscr.com/co2um0" rel="nofollow">http://prntscr.com/co2um0</a>) I use javascript:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
    &lt;meta charset="utf-8"&gt;
    &lt;title&gt;Dialog&lt;/title&gt;
    &lt;script&gt;
      window.addEventListener('beforeunload', function() {
        return false;
      });
    &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;&lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>The dialog will not be shown second time (on current domain during the session) if I click "Leave Page".</p>
  
  <p>The dialog will be shown second time (on current domain during the session) if I click "Stay on Page".</p>
  
  <p>The dialog doesn't have a checkbox like "Don't show again".</p>
  
  <p><strong>Steps to Reproduce:</strong></p>
  
  <ol>
  <li>Add javascript to the page A:</li>
  <li>Open page A in Safari;</li>
  <li>Try to go out from the page A;</li>
  <li>You see dialog "Are you sure you want to leave this page?";</li>
  <li>Click button "Leave Page";</li>
  <li>Go back to the page A;
  7.Try to go out from the page A;</li>
  <li>You don't see the dialog "Are you sure you want to leave this page?";</li>
  </ol>
  
  <p><strong>Expected Results:</strong></p>
  
  <ol start="8">
  <li>You see the dialog "Are you sure you want to leave this page?"</li>
  </ol>
  
  <p><strong>Actual Results:</strong></p>
  
  <ol start="8">
  <li>You don't see the dialog "Are you sure you want to leave this page?".</li>
  </ol>
  
  <p><strong>Reprodused on:</strong></p>
  
  <p>OS X 10.11.6
  Safari 9.1.2</p>
  
  <p><strong>Notes:</strong></p>
  
  <p>Some other browsers has checkbox like "Do not show this message again" in the dialogs (alert dialog for example) when it is shown second time.</p>