Fix UWP XBF files losing subdirectory paths in resources.pri when UseArtifactsOutput is enabled

[Copilot]

When UseUwp=true and UseArtifactsOutput=true, the IntermediateOutputPath moves outside the project directory. The AppX Package Targets compute PRI paths for _GeneratedXbfFiles using %(Filename)%(Extension) for items outside the project directory, dropping subdirectory prefixes (e.g., Styles.xbf instead of Resources\Styles.xbf), causing runtime XamlParseException.

Changes

• Microsoft.NET.Windows.targets: Adds _FixGeneratedXbfFilesLink target that runs AfterTargets="MarkupCompilePass2" when UseUwp=true. For any _GeneratedXbfFiles item outside the project directory (the artifacts case), sets Link to the path relative to $(IntermediateOutputPath):

<Target Name="_FixGeneratedXbfFilesLink"
        AfterTargets="MarkupCompilePass2"
        Condition="'$(UseUwp)' == 'true'">
  <PropertyGroup>
    <_NormalizedIntermediateOutputPath>$([MSBuild]::EnsureTrailingSlash($([System.IO.Path]::GetFullPath('$(IntermediateOutputPath)'))))</_NormalizedIntermediateOutputPath>
  </PropertyGroup>
  <ItemGroup>
    <_GeneratedXbfFiles Update="@(_GeneratedXbfFiles)"
      Condition="'%(Link)' == '' and !%(FullPath).StartsWith(ProjectDirectory)">
      <Link>$([MSBuild]::MakeRelative($(_NormalizedIntermediateOutputPath), %(FullPath)))</Link>
    </_GeneratedXbfFiles>
  </ItemGroup>
</Target>

This ensures Resources\Styles.xbf is correctly emitted into resources.pri rather than just Styles.xbf, matching what the XAML compiler-generated code-behind references via ms-appx:///Resources/Styles.xaml.

• GivenThatWeWantToBuildAWindowsDesktopProject.cs: Adds regression test ItSetsCorrectLinkMetadataOnGeneratedXbfFilesWhenUsingArtifactsOutput that creates a UWP project with UseArtifactsOutput=true, injects simulated _GeneratedXbfFiles items in subdirectories of $(IntermediateOutputPath), and verifies that the _FixGeneratedXbfFilesLink target correctly sets Link metadata to preserve the subdirectory structure (e.g., Resources\Styles.xbf and Views\Home.xbf).

[marcpopMSFT]

@copilot can you add a test for this?

[Copilot]

Pull request overview

Fixes incorrect resource path computation for UWP-generated XBF files when UseArtifactsOutput=true moves IntermediateOutputPath outside the project directory, preventing subdirectory paths from being dropped in resources.pri.

Changes:

• Adds a new MSBuild target (_FixGeneratedXbfFilesLink) that runs after MarkupCompilePass2 for UseUwp=true.
• For _GeneratedXbfFiles outside the project directory, sets Link to the path relative to $(IntermediateOutputPath) to preserve subdirectory structure.

[Copilot]

@copilot can you add a test for this?

Added a regression test in commit c51802b: ItSetsCorrectLinkMetadataOnGeneratedXbfFilesWhenUsingArtifactsOutput in GivenThatWeWantToBuildAWindowsDesktopProject.cs.

The test creates a UseUwp=true project with UseArtifactsOutput=true (via Directory.Build.props), injects fake _GeneratedXbfFiles items in subdirectories of $(IntermediateOutputPath) (simulating what the XAML compiler produces), and verifies that _FixGeneratedXbfFilesLink sets the correct Link metadata preserving subdirectory paths (e.g., Resources\Styles.xbf instead of just Styles.xbf).