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chromium/third_party/blink/web_tests/editing/shadow/selection-shadow-mouse-drag.html 

<!DOCTYPE html>
<style>* { font-family: monospace; }</style>
<div id="host">[<span id="one" slot="one">One</span> <span id="two" slot="two">Two</span>]</div>
<script>
function doDrag(start, end) {
    if (!window.eventSender)
        return;
    eventSender.mouseMoveTo(start.offsetLeft + (start.offsetWidth / 2), start.offsetTop + 3);
    eventSender.mouseDown();
    eventSender.mouseMoveTo(end.offsetLeft + (end.offsetWidth / 2), end.offsetTop + 3);
    eventSender.mouseUp();
}
var host = document.querySelector('#host');
var shadowRoot = host.attachShadow({mode: 'open'});
shadowRoot.innerHTML = '[<slot name="two"></slot> <slot name="one"></slot>]';
var one = document.querySelector('#one');
var two = document.querySelector('#two');
doDrag(two, one);
</script>