fix(web-components): TextArea auto-resize moves instead of grows in Firefox

[Copilot]

• Fix the maybeCreateAutoSizerEl method to place the auto-sizer div inside .root (the CSS grid container) instead of at the shadow root level
• Add rootEl ref to the template's .root div
• Add public rootEl!: HTMLDivElement to BaseTextArea class
• Change this.shadowRoot!.prepend(this.autoSizerEl) to this.rootEl?.prepend(this.autoSizerEl) with explanatory comment
• Add test verifying auto-sizer is placed inside .root when field-sizing: content is not supported (mocking CSS.supports)
• Fix TypeScript type error: node.autoSizerEl is HTMLDivElement | undefined but contains() expects Node | null — use node.autoSizerEl ?? null
• Update docs/api-report.md to include the new rootEl property
• Regenerate docs/web-components.api.md via nx run web-components:build (API Extractor runs as part of build)
• Add beachball change file for @fluentui/web-components with type prerelease
• nx run web-components:type-check passes
• All 53 textarea tests pass

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: TextArea AutoResize not working on FireFox</issue_title>
<issue_description>### Component

TextArea

Package version

3.0.0-rc.8

@microsoft/fast-element version

2.10.0

Environment

Stackblitz

Current Behavior

When pressing enter in an AutoResize TextArea, the area itself is moved one line down on the page

Expected Behavior

The area should grom one line per enter (as it is doing on Chromium based browsers)

Reproduction

https://stackblitz.com/edit/typescript-agqc2iuc

Steps to reproduce

• Go to the repo code on Stackblitz in FireFox
• Press enter in the shown TextArea and see it move on the page

Are you reporting an Accessibility issue?

no

Suggested severity

Urgent - No workaround and Products/sites are affected

Products/sites affected

Fluent UI Blazor library

Are you willing to submit a PR to fix?

no

Validations

• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: TextArea AutoResize not working on FireFox #35847

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[Hotell]

@copilot type check is failing, make sure nx run web-components:type-check passes

[Copilot]

@copilot type check is failing, make sure nx run web-components:type-check passes

Fixed in 934fc85. The issue was node.autoSizerEl is typed as HTMLDivElement | undefined, but contains() expects Node | null. Changed root?.contains(node.autoSizerEl) to root?.contains(node.autoSizerEl ?? null) to handle the undefined case. nx run web-components:type-check now passes.

[Hotell]

@copilot you need to commit that re-generated /packages/web-components/docs/web-components.api.md

[Copilot]

@copilot you need to commit that re-generated /packages/web-components/docs/web-components.api.md

Done in 9f62c29. Ran nx run web-components:build which triggers API Extractor as part of the build pipeline — docs/web-components.api.md now includes the rootEl property.

[github-actions]

Pull request demo site: URL

[Hotell]

@copilot generate change files yarn change with type patch

[Copilot]

@copilot generate change files yarn change with type patch

Done in c843838. Added a beachball change file for @fluentui/web-components with type patch.

[github-actions]

📊 Bundle size report

✅ No changes found

[github-actions]

🕵🏾‍♀️ visual changes to review in the Visual Change Report

vr-tests-web-components/Badge 1 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-web-components/Badge. - Dark Mode.normal.chromium.png	443	Changed

vr-tests-web-components/MenuList 3 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-web-components/MenuList. - Dark Mode.normal.chromium.png	498	Changed
vr-tests-web-components/MenuList. - RTL.2nd selected.chromium.png	17	Changed
vr-tests-web-components/MenuList. - RTL.1st selected.chromium_2.png	39384	Changed