[Bug]: [fullhash] does not update when CSS-only changes occur

[7rulnik]

System Info

System:
    OS: macOS 26.2
    CPU: (16) arm64 Apple M4 Max
    Memory: 7.35 GB / 64.00 GB
    Shell: 5.9 - /opt/homebrew/bin/zsh
  Binaries:
    Node: 22.8.0 - /Users/v7rulnik/.local/state/fnm_multishells/48371_1769509735708/bin/node
    npm: 10.8.2 - /Users/v7rulnik/.local/state/fnm_multishells/48371_1769509735708/bin/npm
  Browsers:
    Chrome: 144.0.7559.97
    Safari: 26.2
  npmPackages:
    @rspack/cli: latest => 1.7.4
    @rspack/core: latest => 1.7.4

Details

When CSS content changes but JavaScript remains unchanged, the [fullhash] placeholder does not update, while [contenthash] correctly updates.

Current behavior:

• CSS [contenthash] correctly updates when CSS changes
• Compilation [fullhash] incorrectly stays the same

Expected behavior:

• Both [contenthash] and [fullhash] should update when CSS content changes

This causes different build outputs to have identical fullhashes.

Webpack comparison: Webpack correctly updates [fullhash] when CSS changes. The reproduction repo includes both Rspack and Webpack tests demonstrating this difference.

Reproduce link

https://github.com/7rulnik/rspack-fullhash-css-repro

Reproduce Steps

1. Clone the repo: git clone https://github.com/7rulnik/rspack-fullhash-css-repro
2. Run the reproduction script:
   cd rspack-fullhash-css-repro/rspack-test
   npm install
   ./reproduce.sh
3. Observe the output:
   - Build 1 produces main.[fullhash].js and main.[contenthash].css
   - CSS is modified (color changes from red to green)
   - Build 2 produces files with the same fullhash but different contenthash

Result:

  BUILD 1:
    [fullhash]:    15d7bc99b1b65226
    [contenthash]: 6fdaa5b099f97dd7

  BUILD 2 (CSS changed, JS unchanged):
    [fullhash]:    15d7bc99b1b65226  ❌ SAME (bug!)
    [contenthash]: 47fc5c9ca2bb0b36  ✅ CHANGED

For comparison, run ./run-both.sh from the root to see Webpack correctly updating [fullhash] for the same scenario.

[7rulnik]

Also question: could it lead to cases when cache doesn't update properly?