Document WinUI 3 testing with MSTest and MTP

[Evangelink]

Summary

Add task-oriented guidance for testing WinUI 3 apps with MSTest on Microsoft.Testing.Platform (MTP), including:

• How to choose unpackaged versus packaged full-trust deployment.
• Required project properties and self-hosted App.OnLaunched setup.
• UITestMethod dispatcher behavior, lifecycle cleanup, and exit-code propagation.
• The exact dotnet run workflow and expected outcome.
• Packaged-app registration, AUMID activation, requirements, experimental status, and the UWP/AppContainer boundary.
• The distinction between classic UWP, modern .NET UWP with UseUwp, and packaged full-trust WinUI 3.
• Focused troubleshooting, TOC registration, and links from existing MSTest and MTP overview pages.

The platform comparison and test-host deployment pages now distinguish supported unpackaged WinUI from the packaged full-trust preview and from classic UWP, modern .NET UWP, and other unsupported AppContainer hosts.

Source breakdown

• Deployment-model decision and common setup: Adapted from microsoft/testfx docs/winui-testing.md.
• Project properties and self-hosted OnLaunched flow: Adapted from the MSTestRunnerWinUI and MSTestRunnerWinUIUnpackaged samples.
• Classic and modern .NET UWP boundary: Adapted from docs/winui-testing.md, the existing MSTest platform guidance, and the BlankUwpNet9App sample.
• Packaged-app registration and AUMID behavior: Adapted from the current Microsoft.Testing.Extensions.PackagedApp source documentation and implementation.
• Published-package availability: Verified against NuGet package 1.0.0-alpha.26377.5; the public package does not yet include Windows AUMID activation, so the article gates that setup on a future supporting preview.
• Troubleshooting, task flow, and cross-page navigation: Newly generated from the verified source behavior and existing dotnet/docs structure.

AI usage and expert review

Created with AI assistance under human direction. The new article narrative, decision table, troubleshooting table, UWP boundary explanation, and cross-page wording are generated/adapted content and require subject-matter expert review.

Validation

• Targeted markdownlint-cli2: 0 errors.
• Frontmatter and ai-usage metadata validated.
• TOC registration and relative Markdown links validated.
• git diff --check passed.
• Multiple technical and documentation-quality review passes completed with blocking findings resolved.

Remaining uncertainty

Full-trust MSIX registration and AUMID activation are implemented in the current microsoft/testfx source but are not present in the latest public Microsoft.Testing.Extensions.PackagedApp prerelease. The packaged activation path therefore couldn't be executed from a public package. The article calls out that release-state limitation and does not direct readers to install an incompatible package.

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Internal previews

File	Preview link
docs/core/testing/microsoft-testing-platform-intro.md	docs/core/testing/microsoft-testing-platform-intro
docs/core/testing/microsoft-testing-platform-test-host-deployment.md	docs/core/testing/microsoft-testing-platform-test-host-deployment
docs/core/testing/test-platforms-overview.md	docs/core/testing/test-platforms-overview
docs/core/testing/unit-testing-mstest-intro.md	docs/core/testing/unit-testing-mstest-intro
docs/core/testing/unit-testing-mstest-running-tests.md	docs/core/testing/unit-testing-mstest-running-tests
docs/core/testing/unit-testing-mstest-winui.md	docs/core/testing/unit-testing-mstest-winui
docs/navigate/devops-testing/toc.yml	docs/navigate/devops-testing/toc

[Copilot]

Pull request overview

Adds a new, task-oriented article that explains how to run MSTest on Microsoft.Testing.Platform (MTP) inside WinUI 3 test hosts (unpackaged and packaged full-trust), and wires that guidance into existing MSTest/MTP documentation and navigation.

Changes:

• Adds a new WinUI 3 testing guide covering deployment model choice, required project properties, self-hosted OnLaunched setup, UI-thread dispatch with UITestMethod, and dotnet run workflow.
• Updates MSTest and MTP overview pages to link to the new WinUI 3 guidance and refreshes metadata.
• Updates the test-platform comparison and test-host deployment docs to clarify unpackaged WinUI support, packaged full-trust preview status, and AppContainer/UWP limitations, plus adds TOC registration.

Show a summary per file

File	Description
docs/navigate/devops-testing/toc.yml	Adds navigation entry for the new WinUI 3 testing article.
docs/core/testing/unit-testing-mstest-winui.md	New end-to-end guidance for hosting and running MSTest via MTP in WinUI 3 apps (unpackaged and packaged full-trust).
docs/core/testing/unit-testing-mstest-running-tests.md	Adds ai-usage metadata and links to the WinUI 3 article from “See also”.
docs/core/testing/unit-testing-mstest-intro.md	Adds ai-usage metadata and updates platform guidance to point to the WinUI 3 article.
docs/core/testing/test-platforms-overview.md	Refines platform selection guidance to distinguish UWP/AppContainer, unpackaged WinUI, and packaged full-trust WinUI preview.
docs/core/testing/microsoft-testing-platform-test-host-deployment.md	Updates packaged-app deployment description/requirements and adds a link to the WinUI 3 article.
docs/core/testing/microsoft-testing-platform-intro.md	Adds ai-usage metadata and links to WinUI 3 hosting guidance from the “Start here” list.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1

[BillWagner]

Thanks again @Evangelink

This LGTM, and I'll now.