fix: respect outer visibility across Web Chat

[OEvgeny]

Fixes #5472

Changelog Entry

• Fixed #5472, ensure proper inheritance from the outside of Web Chat for visibility CSS property when hidden, in PR #5473, by @OEvgeny

Description

This ensures we mostly don't use visibility: visible due to its ability to reveal things if hidden upper in the tree.

Design

Use unset in place of visible as it resets the value to the initial one, which is properly inherited: https://codepen.io/OEvgeny/pen/qEExOdg

Specific Changes

• Changed visibility: visible occurrences to visibility: unset.

-

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

Pull Request Overview

This PR fixes issue #5472 by ensuring that outer visibility styles are properly inherited in Web Chat. The key changes include replacing 'visibility: visible' with 'visibility: unset' in ThumbButton styles, adding an explanatory comment in the ActivityCopyButton animation keyframe, and updating the changelog.

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/component/src/Styles/StyleSet/ThumbButton.ts	Replaced 'visibility: visible' with 'visibility: unset' for proper inheritance
packages/component/src/Styles/StyleSet/ActivityCopyButton.ts	Added comment in keyframes to clarify purpose of using 'visibility: visible' during animation
CHANGELOG.md	Added a changelog entry for issue #5472

Files not reviewed (1)

• packages/fluent-theme/src/components/sendBox/TextArea.module.css: Language not supported

Comments suppressed due to low confidence (2)

packages/component/src/Styles/StyleSet/ThumbButton.ts:58

• Consider verifying that switching to 'unset' fully preserves the desired behavior across all supported browsers, especially for CSS inheritance edge cases.

visibility: 'unset'

packages/component/src/Styles/StyleSet/ThumbButton.ts:69

• Ensure that using 'unset' in this context does not inadvertently remove intended visibility overrides in specific UI states.

visibility: 'unset'