Summary

With CssExtractRspackPlugin({ filename: '[name].[contenthash:8].css' }) + HMR, editing a CSS file never updated the page. Not the off-by-one described in #6869 — permanent, for every edit. Console reported success ("Reload all css"), a network request fired each cycle, and a fresh stylesheet link element was created and parsed, but the content never changed.

Root cause: __webpack_require__.miniCssF is a string literal baked into the runtime chunk at build time. HMR never re-fetches or re-evaluates the runtime chunk, so once the CSS filename's [contenthash] changed on rebuild, the extract-css HMR handler kept resolving the build-1 href forever. A second, compounding bug: even with a fresh href, the stylesheet element lookup (extractCssFindStylesheet) matched by exact href equality, which can never find the currently-installed element once its href has moved on.

Fix:

• crates/rspack_plugin_hmr: the hot-update manifest now carries an optional miniCss map ({chunkId: freshCssFilename}), computed directly from the chunk's own emitted extract-css asset at manifest-generation time — independent of the (unreliable) miniCssF runtime function. Matched by asset ownership (ManifestAssetType::Custom("extract-css"), the same tag rspack_plugin_sri already relies on), not by filename suffix, so a chunk carrying both native CSS and extract-css output — or another plugin's own .css asset — can't have the wrong one picked. Exposed to the runtime via a new HMR_MINI_CSS_FILENAMES global (__webpack_require__.hmrMCF), only emitted when non-empty. Named miniCss (not a generic css) so this extract-css-specific channel stays distinct from any equivalent for the native CSS runtime.
• crates/rspack_plugin_extract_css: runtime-created stylesheet link elements are tagged with a data-webpack-chunk attribute (the owning chunk id, namespaced by output.uniqueName so two independently compiled apps sharing a chunk id on one page can't steal or remove each other's stylesheet — the same identity scheme chunk/module script loading already uses). Lookups during HMR prefer matching on that instead of href. The one exception is the very first HMR update after a page load, where the element was written directly into the HTML by HtmlRspackPlugin and was never tagged — for that case only, the frozen miniCssF href is used as a lookup fallback (it still matches that element's real, unmodified href), while the manifest's fresh href is always used for the fetch itself.
• Lazy chunks are covered too: a chunk that hasn't been imported yet has no element for the HMR handler to swap, so it just retains the fresh filename it learned about. Normal chunk loading, prefetch, and preload now all consult that retained mapping before falling back to miniCssF, so importing a lazy chunk after its CSS changed (the scenario [Bug]: live reload of styles lazy components is broken when using hashes in names #6869 itself is about) resolves the current filename instead of the stale one.
• CssLoadingRuntimeModule::runtime_requirements() (crates/rspack_plugin_extract_css/src/runtime.rs) now also propagates the basic template's weak requirements (CSS_LOADING_BASIC_RUNTIME_REQUIREMENTS.weak, which now includes the HMR_MINI_CSS_FILENAMES read added above) alongside the create-link template's — it previously only propagated the latter. Without this, under experiments.runtimeMode: "rspack" (lexical runtime, where a weak global read compiles to a bare identifier rather than a safe __webpack_require__.x property access), loading an async extracted-CSS chunk with no HMR runtime module present would throw a ReferenceError for the undeclared hmrMiniCssFilenames binding instead of silently falling back to miniCssF. Covered by a new non-HMR runtimeMode: "rspack" config test (see below).

I deliberately left packages/rspack/src/runtime/cssExtractHmr.ts (the loader's module.hot.dispose fallback, used when runtime: false) unchanged. It already coexists with the native hmrC.miniCss handler today — proven by the pre-existing shared-stylesheet-dedup e2e test — and this fix doesn't change its behavior or introduce a new conflict between the two.

While chasing the runtimeMode: "rspack" config test above through the full configCases suite, I also found and fixed a pre-existing, unrelated false positive: css-extract/with-contenthash's "should not contain full hash runtime module" test used a plain substring check for __webpack_require__.h, which incidentally also matches any longer property name starting with the same letter — including the new hmrMCF global added here. Tightened the check to a word-boundary regex so it only matches the actual full-hash accessor.

Related links

• [Bug]: live reload of styles lazy components is broken when using hashes in names #6869 — the bug this fixes.

How it was verified

• Node-only repro (compiler.watch, no browser) confirmed the hot-update manifest now carries the correct, freshly-hashed CSS filename per rebuild.
• e2e regression tests:
  • tests/e2e/cases/css/contenthash-hmr — hashed extract-css filename + HMR, asserts the computed background color actually updates across two consecutive edits (not just an off-by-one), and that exactly one stylesheet element remains after each update.
  • tests/e2e/cases/css/contenthash-hmr-lazy-chunk — edits a lazy chunk's CSS before its first import(), then imports it, and asserts the loaded stylesheet reflects the edit rather than the stale pre-edit filename.
• tests/rspack-test/configCases/runtime/runtime-mode-css-extract-async-chunk — new config test: loads an async extracted-CSS chunk under experiments.runtimeMode: "rspack" with no HMR involved at all, asserting it resolves and doesn't throw the ReferenceError described above. Verified it actually catches the regression: fails with the exact ReferenceError when the weak-requirement propagation fix is reverted, passes with it restored.
• Full configCases suite (Config.part1-3.test.js + RuntimeModeConfig.part1-3.test.js, ~3700 tests each covering both legacy webpack and rspack lexical runtime rendering) — all passing, including the with-contenthash fix above.
• pnpm run test:hot — 124/124 passing (hot-update manifest snapshots updated to reflect the new miniCss field; diffs reviewed — extract-css cases gained miniCss, native-CSS (type: 'css') cases correctly lost the field entirely instead of the old filename-suffix heuristic accidentally matching their asset too).
• Full e2e suite (pnpm run test:e2e) — all passing, including the pre-existing shared-stylesheet-dedup and shared-stylesheet-dedup-link cases (no regression to the two-HMR-owners fix).
• cargo test (rspack_core, rspack_plugin_extract_css, rspack_plugin_hmr, and the full workspace excluding binding crates per contribution guide) — all green.

Checklist

• Tests updated (four new/regenerated test cases; existing hot-update snapshots regenerated and reviewed; one pre-existing test's assertion fixed).
• Documentation updated (not required — no public API/config surface changed).