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chromium/third_party/blink/web_tests/external/wpt/html/webappapis/scripting/events/contextmenu-event-manual.htm 

<!doctype html>
<html>
  <head>
    <title>HTML contextmenu event is a MouseEvent</title>
    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
    <style>#contextmenutarget { width: 100px; height: 100px; background-color: red; }</style>
  </head>
  <body>
    <div id='contextmenutarget'>Trigger context menu in this box.</div>
    <div id="log"></div>
    <script type="text/javascript">
var t = async_test('contextmenu event generated from user action is MouseEvent');
document.querySelector("#contextmenutarget").addEventListener('contextmenu', t.step_func(function (e) {
    assert_equals(e.constructor, window.MouseEvent);
    document.querySelector("#contextmenutarget").style.backgroundColor = "green";
    t.done();
}));
    </script>
  </body>
</html>