[BUG] Popover with a position-area not positioned correctly

[jamesnw]

Describe the bug
Popovers are not positioned correctly, and require an additionalinset: auto declaration to be positioned correctly.

To Reproduce

1. Go to https://codepen.io/jamessw/pen/yyyKYwE?editors=1100
2. In Chromium, both popovers are positioned correctly
3. In polyfilled browsers, inset: auto must be set on the popover for it to be positioned correctly, as in the top button/popover.

Expected behavior
Users shouldn't need to add additional styles for the polyfill to work. We should either apply it, or find a way to not need to apply it.

Browsers tested on
Safari, Firefox