Bug 259261

When a [popover] element that is a descendant of an open [popover] element exists within a shadow DOM, the topmost clicked [popover] element (https://html.spec.whatwg.org/multipage/popover.html#topmost-clicked-popover) calculation is no done with the composed path which causes the descendant [popover] element to close the ancestor [popover] element at open time.

You can repro this here: https://codepen.io/Westbrook/pen/BaGrdoq

With the following code:

<button popovertarget="popover" popovertargetaction="toggle">Toggle</button>
<div id="popover" popover>
  <template shadowrootmode="open">
    Outter
    <button popovertarget="popover-2" popovertargetaction="toggle">Toggle</button>
  <div id="popover-2" popover>
    Inner
  </div>
  </template>
</div>