[benjaminkincses]

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• AI was used: Muse (Meta) agent, pair-programmed with the contributor
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Description

css.lightningcss.cssModules (e.g. { animation: false }) was silently dropped whenever css.modules resolved to true (or was unset). In both transformWithLightningCSS and bundleWithLightningCSS (packages/css/src/lightningcss.ts), ...options.lightningcss was spread first and then cssModules: options.cssModules overwrote it, so keyframes were always renamed - desyncing them from CSS-variable animation references (lightningcss known limitation).

The fix merges instead of overwriting: object-form css.modules still wins over nested lightningcss options, and object-form lightningcss.cssModules is preserved when css.modules is just true (or unset).

Linked Issues

Closes #1083

Additional context

Regression tests added to packages/css/src/lightningcss.test.ts covering both transform and bundle paths: keyframes are kept global with animation: false, still renamed by default, and object-form configs merge.

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commit: b9be58c

[sxzz]

Thanks for investigating this and adding regression tests. Closing in favor of #1103.

The merge here also applies lightningcss.cssModules when the internally resolved cssModules value is undefined or false. That can enable module scoping for ordinary CSS and during final minification, potentially renaming classes a second time and leaving the CSS out of sync with the JavaScript exports.

#1103 merges the options in resolveCssModulesConfig, after checking whether the file is a CSS module. This preserves tsdown's naming overrides and keeps module scoping out of ordinary CSS and final minification. It also adds regression coverage for CSS/SCSS, minification, naming precedence, and disabled CSS Modules.