feat(css): resolve bare @import specifiers to node_modules as fallback

[murataslan1]

Summary

Fixes #295074 — CSS @import "some-module/style.css" now resolves to node_modules/some-module/style.css as a fallback when the local file doesn't exist.

Problem

CSS @import supports relative paths without leading ./, and VS Code resolves them correctly as relative paths. However, when using a bundler like Vite or Webpack, it's common to import from node_modules with a bare specifier:

@import "some-module/style.css";

Currently, Ctrl+Click on such imports doesn't work because VS Code only resolves the path relative to the current file directory. The ~ prefix workaround (@import "~some-module/style.css") works but is non-standard and not used by most modern bundlers.

Solution

Added a post-processing step in the CSS language server's document link handler:

1. After findDocumentLinks2() returns links, check if each link target actually exists on disk
2. For bare specifiers (not starting with ., /, ~, or containing ://), if the local file doesn't exist, try resolving against node_modules/ in the workspace root
3. If the node_modules/ version exists, use it as the link target

Rules:

• ✅ @import "foo/style.css" → tries node_modules/foo/style.css if local doesn't exist
• ✅ @import "./foo.css" → keeps relative resolution (no fallback)
• ✅ @import "/foo.css" → keeps absolute resolution (no fallback)
• ✅ @import "~foo/style.css" → keeps existing tilde behavior (no fallback)
• ✅ Local file always takes priority over node_modules

Files Changed

File	Change
server/src/utils/nodeModuleLinks.ts	New: resolveNodeModuleLinks() utility function
server/src/cssServer.ts	Use node_modules fallback in onDocumentLinks handler
server/src/test/links.test.ts	3 test cases: bare module fallback, local priority, relative skip
server/test/linksTestFixtures/node_modules/foo/hello.html	Test fixture file

[Copilot]

Pull request overview

Adds a node_modules fallback for CSS document links so that bare @import "pkg/path.css"-style specifiers can be Ctrl+Clicked and resolved when the relative file doesn’t exist, aligning VS Code navigation with common bundler behavior.

Changes:

• Introduce resolveNodeModuleLinks() to post-process CSS document links and attempt node_modules/<specifier> resolution as a fallback.
• Wire the post-processing into the CSS language server’s onDocumentLinks handler.
• Add test coverage and fixtures for bare specifier fallback, local-priority behavior, and “no fallback for relative” behavior.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
extensions/css-language-features/server/src/utils/nodeModuleLinks.ts	New utility that checks link existence and optionally rewrites targets to node_modules/… for bare specifiers.
extensions/css-language-features/server/src/cssServer.ts	Applies the new link post-processing in the document links handler.
extensions/css-language-features/server/src/test/links.test.ts	Adds unit tests for the new fallback behavior.
extensions/css-language-features/server/test/linksTestFixtures/node_modules/foo/hello.html	Adds fixture file used by the new tests.

[Copilot]

The doc comment says this utility is for CSS @import paths, but the implementation doesn't verify that a DocumentLink came from an @import statement (it will run for url(...) links too, since it only checks refText). Either tighten the logic to only apply to @import links (e.g. inspect surrounding text near link.range), or update the comment/PR intent so callers understand the broader behavior change.

[Copilot]

requestService.stat() does not consistently throw on missing files in this codebase (e.g. getNodeFSRequestService().stat resolves with type: FileType.Unknown on ENOENT). Using try/catch here will treat missing files as existing, preventing the node_modules fallback (and may also incorrectly accept missing nodeModuleTarget). Consider checking the returned FileStat.type (or otherwise explicitly treating Unknown as non-existent) in both stat calls instead of relying on exceptions.