fix(css): keep the emitted .map asset aligned when css.inject prepends the import

[NullVoxPopuli]

css.inject prepends import './style.css'; to styled chunks in generateBundle and compensates chunk.map with a ; mappings prefix. that compensation is correct — but rolldown materializes the companion .map asset before generateBundle runs and doesn't propagate chunk.map mutations into it (rolldown/rolldown#10676). so the emitted map is one line behind the emitted code, and every mapping in every css-injecting chunk points one line early.

found this while testing sourcemap alignment for ember-scoped-css's tsdown build — every component in the bundle was misaligned, cascading (in this kind of visualization, each statement highlights the line above itself).

the fix: extract the duplicated prepend logic into prependCssImport(), which also refreshes the .map asset from the mutated chunk.map. works with today's rolldown; harmless if rolldown starts syncing the asset itself.

the test decodes the mappings VLQ inline (no new dependency) and asserts export const value = 42 traces to its exact original line through the injected import. red without the fix, green with it.

---

Disclosure: this investigation, fix, and write-up are AI-assisted exploration (Claude Fable 5 via Claude Code), directed and reviewed by me.

🤖 Generated with Claude Code

[netlify]

✅ Deploy Preview for tsdown-main ready!

Name	Link
🔨 Latest commit	9dcba0c
🔍 Latest deploy log	https://app.netlify.com/projects/tsdown-main/deploys/6a7e42d90dba0b0008dc3072
😎 Deploy Preview	https://deploy-preview-1049--tsdown-main.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pkg-pr-new]

Open in StackBlitz

tsdown

pnpm add https://pkg.pr.new/tsdown@1049 -D

npm i https://pkg.pr.new/tsdown@1049 -D

yarn add https://pkg.pr.new/tsdown@1049.tgz -D

create-tsdown

pnpm add https://pkg.pr.new/create-tsdown@1049 -D

npm i https://pkg.pr.new/create-tsdown@1049 -D

yarn add https://pkg.pr.new/create-tsdown@1049.tgz -D

@tsdown/css

pnpm add https://pkg.pr.new/@tsdown/css@1049 -D

npm i https://pkg.pr.new/@tsdown/css@1049 -D

yarn add https://pkg.pr.new/@tsdown/css@1049.tgz -D

@tsdown/exe

pnpm add https://pkg.pr.new/@tsdown/exe@1049 -D

npm i https://pkg.pr.new/@tsdown/exe@1049 -D

yarn add https://pkg.pr.new/@tsdown/exe@1049.tgz -D

tsdown-migrate

pnpm add https://pkg.pr.new/tsdown-migrate@1049 -D

npm i https://pkg.pr.new/tsdown-migrate@1049 -D

yarn add https://pkg.pr.new/tsdown-migrate@1049.tgz -D

commit: 9dcba0c

[Copilot]

Pull request overview

Fixes sourcemap misalignment when css.inject prepends an import during generateBundle by ensuring the emitted companion .map asset is updated to match the mutated chunk.map (working around current rolldown behavior).

Changes:

• Extracted CSS import-prepend logic into prependCssImport() and refreshed the emitted ${chunk.fileName}.map asset after adjusting mappings.
• Added a sourcemap alignment regression test for css.inject that decodes VLQ mappings and verifies a specific statement maps back to the correct original line.
• Added a new snapshot covering the injected import, emitted map, and CSS output.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/css.test.ts	Adds a regression test that decodes mappings and asserts emitted map alignment after injected CSS import.
tests/snapshots/sourcemap/css-inject-keeps-the-emitted-map-aligned-with-the-injected-import.snap.md	Snapshot verifying the injected import and updated sourcemap mappings.
packages/css/src/plugin.ts	Refactors prepend logic into prependCssImport() and updates the .map asset to reflect chunk.map mutations.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (1)

tests/css.test.ts:1871

• generatedLine becomes 0 if findIndex() fails (returns -1), which makes the subsequent sourcemap assertion fail with a less helpful message. Capturing the index separately and asserting it was found improves test diagnostics and avoids silently passing an invalid line number into originalLineFor().

      const generatedLine =
        code.split('\n').findIndex((line) => line.includes('const value')) + 1