CSS hashes change based on absolute filename in pnpm monorepo

[sfriedel]

Describe the bug

I've recently had problems with some component snapshot tests that fail on CI because Svelte generates different CSS hashes on the CI runner than on my local machine.

This seems to be caused by the introduction of filename based CSS hashing in #16740. It appears that in some cases the cssHash function is given an absolute filename which then leads to different hashes on CI vs. dev because the parent directories are different.
It should be noted that i was only able to reproduce when using internal workspace dependencies (using the workspace: protocol) in a pnpm monorepo.

It would be great if the cssHash would not change based on environment details like parent directories and reading #16636 it appears that this was the intention.

Reproduction

• open the reproduction example on Stackblitz
• wait until the pnpm --dir webapp test command finishes in the terminal
• the output should contain a cssHash filename: line followed by an absolute path. the line is produced by an overridden compilerOptions.cssHash function in webapp/svelte.config.js

Logs

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm

Severity

annoyance