special case anchored-position for top-layer elements

[keithamus]

When calculating the positioning for an anchored position element, the current algorithm traverses the tree to find "positioned parents"; that is parents with a non-default overflow or position.

This is acceptable for most elements, but native <dialog> and elements with popover= get reflected onto the :top-layer element, which is adjacent to the document, and therefore layout should be calculated using the main viewport, not any overflow/position parents.

This PR adds a check to special case such elements, by checking if the element is a dialog, or by checking if the :popover-open psuedo class is active. Sadly there is no pseudo selector to detect whether or not an element is on the top layer, so we have to check for likely candidates instead.

Fixes primer/view_components#2000

[changeset-bot]

🦋 Changeset detected

Latest commit: bddd6c5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/behaviors	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR