[OvesN]

Fixes #13748

Context

AbsolutePath.Value was not guaranteed to be fully qualified when the struct was constructed via AbsolutePath(string, AbsolutePath). On Windows, Path.Combine(basePath, path) returns the second argument unchanged when it is rooted-but-not-fully-qualified (\foo root-relative, X:foo drive-relative), so Value could end up as a non-absolute string. A later GetCanonicalForm() call would then resolve it via System.IO.Path.GetFullPath, which consults process-global state (current drive, per-drive cwd) — breaking determinism and multithreaded task isolation.

Changes

• AbsolutePath(string, AbsolutePath) anchors the two Windows rooted-but-not-fully-qualified shapes against basePath at construction time, via a new private MakeFullyQualified helper:
  • Root-relative (\foo, /foo) → re-rooted under basePath's root using Path.GetPathRoot.
  • Drive-relative (X:foo) → drive dropped, remainder anchored under basePath via Path.Combine.
• Anchoring uses pure string operations rather than Path.GetFullPath(path, basePath) so the un-normalized form is preserved. GetCanonicalForm() remains the single normalization step.
• The fix is conditioned on #if NETFRAMEWORK || NET because Path.IsPathFullyQualified is not available in netstandard2.0. The netstandard2.0 build is a compat surface only and never runs at runtime in MSBuild.

Testing

AnchorsToBasePath_NotProcessState covers:

• Root-relative and drive-relative inputs against a drive-letter base path (same and different drive).
• Root-relative and drive-relative inputs against UNC and DOS-device base paths.
• Pass-through verification for fully qualified inputs (drive, UNC, DOS device) — confirms they are not modified.
• Plain relative inputs (foo, sub\file.txt) anchored to basePath — confirms CWD is never consulted.

[Copilot]

Pull request overview

This PR addresses non-deterministic Windows path canonicalization in Microsoft.Build.Framework.AbsolutePath by ensuring rooted-but-not-fully-qualified inputs (e.g. \foo, X:foo) are anchored to the provided base path rather than process-global state (current drive / per-drive CWD), improving multithreaded task isolation.

Changes:

• Updates AbsolutePath(string, AbsolutePath) to pre-anchor Windows rooted-but-not-fully-qualified shapes during construction via a helper.
• Adds a Windows-only regression theory covering root-relative, drive-relative, UNC, and device-path base paths to ensure determinism for both Value and GetCanonicalForm().
• (Review finding) The anchoring logic is currently excluded from the .NETStandard TFM, leaving the original behavior there.

Reviewed changes

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

File	Description
src/Framework/PathHelpers/AbsolutePath.cs	Adds Windows anchoring logic to avoid Path.GetFullPath resolving against process-global state for certain rooted-but-not-fully-qualified inputs.
src/Framework.UnitTests/AbsolutePath_Tests.cs	Adds Windows-only regression coverage asserting construction and canonicalization are anchored to the supplied base path.

[JanProvaznik]

the base idea is right
only must fix is the static