[BUG] Popover finds different anchor

[jamesnw]

Describe the bug
An anchored popover is finding a different anchor element than the Chromium implementation.

To Reproduce

1. Go to https://codepen.io/jamessw/pen/yyyBwao and open the popover
2. In Chromium, the resolved anchor is the last anchor in the tree
3. The polyfill resolves the anchor as the popover button

Expected behavior
Include relevant parts of the spec, and a clear and concise description of what you expected to happen.

Browsers tested on
Firefox 124 (has popover polyfilled as well)
Firefox 137
Chromium 135