Fix ScopedCss incrementalism to detect CssScope metadata changes

[javiercn]

Summary

This PR fixes the ScopedCss build pipeline to properly regenerate scoped CSS files when the CssScope metadata changes.

Problem

When users specify a custom CssScope metadata on their scoped CSS files (via <None Update="..." CssScope="custom-scope" />), changing that value did not trigger regeneration of the scoped CSS output. This was because MSBuild's Inputs/Outputs incrementalism only tracks file timestamps, not item metadata like CssScope.

Fixes #50646

Solution

1. Added _CreateScopedCssScopeCache target - Creates a cache file containing all CssScope values using WriteLinesToFile with WriteOnlyWhenDifferent="true". This file only updates when scope values actually change.

2. Updated _ProcessScopedCssFiles target - Added the cache file to the target's Inputs, so MSBuild's incrementalism detects when scopes change.

3. Set SkipIfOutputIsNewer="false" on RewriteCss - Since target-level incrementalism now correctly determines when to run, the task's internal timestamp check is disabled to avoid conflicts.

Testing

• Added integration test Build_RegeneratesScopedCss_WhenCssScopeMetadataChanges that verifies:
  • Initial build uses auto-generated scope
  • Adding custom CssScope regenerates files with new scope
  • Changing CssScope value regenerates files again
  • Subsequent builds without changes remain incremental

[Copilot]

Pull request overview

This PR aims to fix the ScopedCss build pipeline to properly regenerate scoped CSS files when the CssScope metadata changes. The solution introduces a cache file mechanism to track CssScope values and integrates it with MSBuild's incrementalism.

Key Changes:

• Added _CreateScopedCssScopeCache target to create a cache file containing CssScope metadata values
• Updated _ProcessScopedCssFiles target to include the cache file in its Inputs attribute
• Set SkipIfOutputIsNewer="false" on RewriteCss task to rely on target-level incrementalism
• Added comprehensive integration test verifying the fix

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/StaticWebAssetsSdk/Targets/Microsoft.NET.Sdk.StaticWebAssets.ScopedCss.targets	Adds new _CreateScopedCssScopeCache target and updates _ProcessScopedCssFiles to track CssScope metadata changes through a cache file
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/ScopedCssIntegrationTests.cs	Adds regression test Build_RegeneratesScopedCss_WhenCssScopeMetadataChanges that verifies files regenerate when CssScope changes and remain incremental when unchanged

[Copilot]

The property _ScopedCssScopeCacheFile is defined inside the _CreateScopedCssScopeCache target but referenced in the Inputs attribute of _ProcessScopedCssFiles. MSBuild evaluates target attributes like Inputs and Outputs during the evaluation phase before any targets execute, so _ScopedCssScopeCacheFile will be undefined when this evaluation occurs, breaking the incrementalism mechanism.

Both _ScopedCssScopeCacheFile and _ScopedCssIntermediatePath should be defined in a PropertyGroup outside of any target, similar to how _GeneratedStaticWebAssetsInputsCacheFile and _StaticWebAssetsIntermediateOutputPath are defined in Microsoft.NET.Sdk.StaticWebAssets.5_0.targets. These definitions can then be removed from within the targets to avoid redundancy.