UI: Backport compat overlay fixes to WordPress 7.1

[ciampo]

Backports #80278 and #80310 to wp/7.1.

What?

Manually backports two @wordpress/ui compat overlay fixes:

• Default Popover.Portal to the compat overlay slot when the host opts in.
• Keep the compat overlay slot available to assistive technologies while an @wordpress/components Modal is open.

Why?

Both fixes landed on trunk after the WordPress 7.1 branch split. Without them, a controlled UI Popover can stack below a host overlay, and a persistent compat slot can be hidden from assistive technologies by Modal isolation.

How?

Cherry-picks the two merged changes as separate commits. The only manual conflict resolution keeps both changelog entries in the 7.1 branch's current Unreleased section.

Testing Instructions

1. CI checks pass
2. Run npm run storybook:dev and open Playground → Debug fixtures → WP Compat Overlay Slot.
3. In both stories, open the UI Popover and its nested Autocomplete. Confirm they render above the host overlay.
4. While the Modal story is open, confirm [data-wp-compat-overlay-slot] has aria-hidden="false".

Testing Instructions for Keyboard

1. Open each host overlay with Enter or Space.
2. Tab to the UI Popover trigger and open it with Enter or Space.
3. Tab to the nested Autocomplete, type to filter, and use the arrow keys to navigate.
4. Press Escape to close the active overlay and confirm focus remains within the expected overlay flow.

Use of AI Tools

Authored with Codex assistance.

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

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[github-actions]

Size Change: +284 B (0%)

Total Size: 7.72 MB

📦 View Changed

Filename	Size	Change
build/modules/boot/index.min.js	49.4 kB	+31 B (+0.06%)
build/modules/content-types/index.min.js	164 kB	+31 B (+0.02%)
build/scripts/block-directory/index.min.js	44.1 kB	+17 B (+0.04%)
build/scripts/block-editor/index.min.js	422 kB	+102 B (+0.02%)
build/scripts/components/index.min.js	271 kB	+23 B (+0.01%)
build/scripts/edit-post/index.min.js	52 kB	+22 B (+0.04%)
build/scripts/edit-site/index.min.js	303 kB	+20 B (+0.01%)
build/scripts/editor/index.min.js	496 kB	+19 B (0%)
build/scripts/media-utils/index.min.js	121 kB	+19 B (+0.02%)

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[aduth]

Verified that this is an effective backport of the linked pull requests compatible with the wp/7.1 branch, and that the rationale tracks that this would be a bug fix we would want to include in the WordPress 7.1 release 👍

[t-hamano]

I just cherry-picked this PR to the release/23.6 branch to get it included in the next release: 1c492bd