Details

postcss-loader continues to produce invalid source map paths on Windows, after #224 was closed.
Said issue was pointing to css-loader as a culprit.
css-loader was fixed.
postcss-loader was not.

Reproduction (Code)

The following webpack loader sequence produces valid source map paths:

[
  MiniCssExtract.loader,
  "css-loader",
  "sass-loader"
]

The following webpack loader sequences produces INVALID paths:

[
  MiniCssExtract.loader,
  "css-loader",
  "postcss-loader",
  "sass-loader"
]

I have also written a small custom loader which passes through the content and sourcemap from the previous loader, while logging the sourcemap sources array with a simple console.log() call.

Placing said logger on top of sass-loader shows paths relative to the webpack project root. Placing said logger on top of postcss-loader shows the same type of corrupted paths as mentioned in #224.

Environment