Fix handling of absolute config files in upgrade tool

[philipp-spiess]

Closes #15220

This PR fixes an issue where the upgrade tool would not be able to load some JavaScript config files across different drive letters on Windows.

The issue in detail is that path.relative(…) tries to build a relative path but if the file is inside the same folder, it won't start the relative path with a ./ so we manually appended it in case that it isn't there. The issue on Windows specifically is that file.relative(…) can also return a legit absolute path, e.g. when the file is on a different drive. In this case we obviously don't want to prefix a path with ./.

Test plan

To reproduce this issue, I checked out a Tailwind v3 project on a different drive letter than my Windows installation. In that case, I was adding a repo inside D: while npm was installed in C:. I then run npx @tailwindcss/upgrade to reproduce the issue.

The fix was validated with a local bun run of the upgrade tool:

[RobinMalfait]

Such a simple fix, love it!

[kudziajaroslaw98]

Problem still persist.
I removed npx cache, removed package, npm upgrade, installed it and ran again.

≈ tailwindcss v4.0.0                                                                                                                                                                                                                                                                                                                                    

│ Searching for CSS files in the current directory and its subdirectories… 

│ ↳ Linked `.\tailwind.config.ts` to `.\src\app\globals.css` 

│ Migrating JavaScript configuration files…

│ ↳ Could not load the configuration file: Can't resolve '././D:\<HappyPath>\tailwind.config.ts' in 'C:\<EvenHappierPath>\npm-cache\_npx\001fd1a00a7ecb8d\node_modules\@tailwindcss\upgrade\dist' 

[msoler75]

Same problem here:

PS D:\projects\tseyor\laravel_inertia> npm cache clean --force
npm warn using --force Recommended protections disabled.
PS D:\projects\tseyor\laravel_inertia> 
PS D:\projects\tseyor\laravel_inertia> npx @tailwindcss/upgrade@next
≈ tailwindcss v4.0.0

│ Searching for CSS files in the current directory and its subdirectories… 

│ Migrating JavaScript configuration files… 

│ ↳ Could not load the configuration file: Can't resolve 
│   '././D:\projects\tseyor\laravel_inertia\tailwind.config.js' in
│   'C:\Users\Marcel\AppData\Local\npm-cache\_npx\0044b4041b3b52e1\node_modules\@tailwindcss\upgrade\dist'

PS D:\projects\tseyor\laravel_inertia> 

[philipp-spiess]

@kudziajaroslaw98 @msoler75 hey! Sorry for the confusion but we only release v4.0.1 right now. Mind updating and trying again?

[thecrypticace]

It's not quite out yet — waiting for the build to finish and npm to publish it. Should be like… 10–15m from now.

[thecrypticace]

It's out now 👍

[dimitrov93]

i got the same issue, any fix? :(

[kudziajaroslaw98]

I don't know if it's me but it's not working still.
The problem changed tho.

≈ tailwindcss v4.0.1

│ Searching for CSS files in the current directory and its subdirectories…

│ ↳ Linked `.\tailwind.config.ts` to `.\src\app\globals.css`

│ Migrating JavaScript configuration files…

│ ↳ Could not load the configuration file: e is not a function

[marcopedone]

Same problem here

≈ tailwindcss v4.0.1

│ Searching for CSS files in the current directory and its subdirectories…

│ ↳ Linked `.\tailwind.config.ts` to `.\src\styles\globals.css`

│ Migrating JavaScript configuration files…

│ ↳ Could not load the configuration file: v is not a function

[philipp-spiess]

@marcopedone @kudziajaroslaw98 This seems to be a different error, mind creating a new issue with a repro so we can take a look?

[al-imam]

Issues is not resolved but pull request it closed 😂
Merged without proper testing.