feat: watch the folders a check takes its files from

[alexander-akait]

Summary

A file webpack never builds has no watcher of its own until it exists, so adding one changed nothing until something else did — #330 watches the files a check read, and a file that is not there yet is not one of them. The folders a check globs, and the ones a tsconfig.json include covers, now go to webpack as contextDependencies, and the paths an import resolved to nothing through as missingDependencies, so writing the file an import names rebuilds and clears the error. No watcher of our own: this is the same API require.context and webpack's resolver use.

Webpack rebuilds on a change anywhere under a folder it watches, so a folder is watched only when the whole of it can be. Two rules, both measured:

• It must not hold what the build writes. With output.path under a watched folder, one edit produced 3 builds instead of 2 — every build's own output is the next build's trigger.
• It must not hold what the check leaves out. Watching a project root with node_modules in it cost +85 MB RSS, against +12 MB with node_modules ignored and +1 MB for a src folder.

So typescript gets this from its tsconfig.json — its include roots are watched whatever files says, which is what fork-ts-checker does — and a glob check gets it when files points at your sources rather than at the project root. Rebuild time and RSS on a 300-file project are unchanged (typescript 49 → 52 ms, 258 → 259 MB, both within the run-to-run spread).

What kind of change does this PR introduce?

feat

Did you add tests for your changes?

Yes — test/stylelint/unbuilt.test.js (a file appearing with nothing else changing), test/typescript/watch.test.js (a file include covers appearing, and the path an unresolved import is watched through), and test/stylelint/directories.test.js for the rules above (watched, output inside, excluded child, folder that is not there).

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Documented in README.md under files.

Use of AI

Claude Code wrote the change, the tests and this description, and ran the measurements above; I reviewed all of it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GzZci4NQeiqwdrVfd7dGXy

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Generated by Claude Code