fix(rollup-plugin-import-css): register CSS files with rollup watcher

[joshfarrant]

Overview

Primer's internal rollup-plugin-import-css plugin reads CSS files via fs.readFile in its transform hook but never calls this.addWatchFile() to register them with rollup's file watcher. CSS-only edits are invisible in watch mode, while TSX/TS changes trigger rebuilds fine.

One-line fix: add this.addWatchFile(sourceId) before reading the file. Verified end-to-end in a github-ui Codespace — CSS changes now trigger rollup rebuilds and propagate through to github-ui's node_modules.

This primarily affects the primerize workflow in github-ui, where developers editing Primer CSS modules in watch mode see no rebuild until they also touch the importing TSX file.

Rollout strategy

• None — rollup-plugin-import-css is a private workspace package, not published to npm. No consumer-facing change.

Changeset skipped

The affected package is private and internal ("private": true, version 0.0.0). Nothing published, no version bump needed.

[changeset-bot]

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Latest commit: 8c0e506

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[Copilot]

Pull request overview

This PR fixes Rollup watch-mode behavior in the internal rollup-plugin-import-css workspace package by ensuring the original CSS source file is registered with Rollup’s file watcher, so CSS-only edits correctly trigger rebuilds.

Changes:

• Register the resolved .css file path via this.addWatchFile(sourceId) in the plugin transform hook for .css.js virtual modules.

Show a summary per file

File	Description
packages/rollup-plugin-import-css/src/index.ts	Adds addWatchFile for the underlying CSS file so watch mode rebuilds on CSS changes.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0