bug: ensure Announce components work without pre-existing text node

[khiga8]

Closes #5566, https://github.com/github/accessibility-audits/issues/10031, https://github.com/github/accessibility-audits/issues/10017

Right now, announcements are not being triggered when the announce components (Announcement, AriaStatus, AriaAlert) go from being empty, to having text. See linked issue for broken examples.

The announcements do work in instances where these components have pre-existing text that gets updated.

Upon placing a debugger, I noticed we're not hitting the MutationObserver's callback in the failing scenario (where we go from empty to no text). This is because we need to set childList: true to detect when text nodes are added/removed. The current config, subtree: true and characterData: true, will only detect when existing text nodes are updated.

• MDN #childList:

Set to true to monitor the target node (and, if subtree is true, its descendants) for the addition of new child nodes or removal of existing child nodes. The default value is false.

Changelog

Changed

• Ensures that announcement are triggered when content is added to Announce, AriaAlert, AriaStatus without pre-existing text.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	104.2 KB (-0.05% 🔽)
packages/react/dist/browser.umd.js	104.58 KB (+0.02% 🔺)