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chromium/third_party/blink/web_tests/fast/dom/shadow/mouse-click-mouseup-listener-update-distribution-crash.html 

<!DOCTYPE html>
<div>crbug.com/417064 minimal reproduction case for hitting assertion check</div>
<span id='clickme'>Click me!</span>
<script>
if (window.testRunner)
    testRunner.dumpAsText();

var clickme = document.querySelector('#clickme');

clickme.addEventListener('mouseup', function() {
    clickme.attachShadow({mode: 'open'});
}, false);

if (window.eventSender) {
    eventSender.mouseMoveTo(clickme.offsetLeft + clickme.offsetWidth / 2, clickme.offsetTop + clickme.offsetHeight / 2);
    eventSender.mouseDown();
    eventSender.mouseUp();
}
</script>