[Bug] PigmentCSS. mixins are not skipped in shouldSkipGeneratingVar

[o-alexandrov]

Steps to reproduce

Link to live example: packages/mui-material/src/styles/createMixins.js

Current behavior

The file createMixins contains invalid CSS to place in a CSS variable.
PigmentCSS adds this invalid CSS as the value of the generated --mui-mixins... CSS variables.

Expected behavior

mixins is skipped in cssVariables.shouldSkipGeneratingVar by default

Context

I stumbled on this issue, when using a minifier on the CSS.
If you refer to createMixins.js you can see you have @media queries in the keys. It breaks the css variables names.

This is what the CSS minifier highlighted:

warnings when minifying css:
▲ [WARNING] Expected ":" [css-syntax-error]

    <stdin>:1:5100:
      1 │ ...eight:56px;--mui-mixins-toolbar-@media (min-width:0px)-@media (o...
        │                                    ^
        ╵                                    :


▲ [WARNING] Expected ":" [css-syntax-error]

    <stdin>:1:5191:
      1 │ ...eight:48px;--mui-mixins-toolbar-@media (min-width:600px)-minHeig...
        │                                    ^
        ╵                                    :

Your environment

npx @mui/envinfo

System:
    OS: macOS 15.0.1
  Binaries:
    Node: 22.10.0 - /opt/homebrew/bin/node
    npm: 10.9.0 - /opt/homebrew/bin/npm
    pnpm: 9.9.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 130.0.6723.70
    Edge: 112.0.1722.39
    Safari: 18.0.1
  npmPackages:
    @emotion/react:  11.13.3 
    @emotion/styled:  11.13.0 
    @mui/base:  5.0.0-beta.59 
    @mui/core-downloads-tracker:  6.1.4 
    @mui/icons-material:  6.1.4 
    @mui/lab:  6.0.0-beta.12 
    @mui/material:  6.1.4 
    @mui/material-pigment-css:  6.1.4 
    @mui/private-theming:  6.1.4 
    @mui/styled-engine:  6.1.4 
    @mui/system:  6.1.4 
    @mui/types:  7.2.18 
    @mui/utils:  6.1.4 
    @pigment-css/react:  0.0.24 
    @pigment-css/vite-plugin:  0.0.24 
    @types/react:  18.3.11 
    react:  18.3.1 
    react-dom:  18.3.1 
    typescript:  5.5.4 

Search keywords: pigmentcss, mixins, shouldSkipGeneratingVar

[brijeshb42]

@siriwatknp I think there might be an already existing solution for this. Do you have any idea ?