[与世无争剑宇子 (yushiwuzhengjianyuzi)]

Fixes #5505

Summary

The resize bars use --colorNeutralStroke2 with an opacity of 0.4, which makes them too faint against the surrounding surface and fails the 3:1 non-text contrast target.

This change:

• Uses the accessible neutral stroke palette for the default, hover, and active bar states.
• Removes the 0.4 opacity and changes the hover opacity to 1.
• Uses --colorNeutralBackground1 for the resize-handle icon so it remains legible against the darker bar.
• Keeps the CSS custom properties documented in the MultiSplitter demo page in sync.

Reviewer notes

This is a visual/accessibility-only change. The updated colors rely on the existing Fluent design tokens and should adapt to both light and dark themes.

Test plan

• dotnet build src/Core/Microsoft.FluentUI.AspNetCore.Components.csproj — passed with 0 warnings and 0 errors.
• dotnet test tests/Core/Components.Tests.csproj --filter "FullyQualifiedName~Splitter" — 28 passed, 0 failed.
• git diff --check — passed.

Checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

AI contribution declaration

AI assistance is allowed; fully AI-generated or autonomous pull requests without meaningful human review are not accepted. See the AI contribution policy.

• I have read and followed the AI contribution policy.
• I have personally reviewed and understood all changes, tested them, and take responsibility for this contribution.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

AI assistance: Codex was used to investigate the issue, implement the CSS/documentation changes, and run the listed validation. The author-review checkbox is intentionally left unchecked pending the author's review.

[Copilot]

🔵 Needs a closer look

Human author review remains pending, and rendered contrast has not been independently verified in both light and dark themes.

No open issues 🎉

What changed in this PR

Updates FluentMultiSplitter resize-bar styling to address the 3:1 non-text contrast requirement in #5505.

Changes:

• Uses accessible neutral stroke tokens for default, hover, and active bar states, with full opacity.
• Uses the neutral background token for resize-handle icons.
• Synchronizes the documented CSS custom-property defaults.

File	Description
src/​Core/​Components/​Splitter/​FluentMultiSplitter.razor.css	Updates resize-bar colors, opacity, and icon colors.
examples/​Demo/​FluentUI.Demo.Client/​Documentation/​Components/​Splitter/​MultiSplitter.md	Documents the updated defaults and new hover-color property.

_{Review effort: Balanced}