`@source` silently ignores paths excluded by an allow-list `.gitignore`

[GrimLink]

What version of Tailwind CSS are you using?

v4.1.13

What build tool (or framework if it abstracts the build tool) are you using?

@tailwindcss/cli 4.1.13

What version of Node.js are you using?

v24.14.0

What browser are you using?

N/A

What operating system are you using?

macOS 26.3.1

Reproduction URL

https://github.com/GrimLink/tailwind-gitignore-bug

Describe your issue

When a project uses an allow-list .gitignore pattern (starting with * to exclude everything, followed by ! rules to allow specific paths), Tailwind v4 silently ignores @source directives that point to git-ignored directories. No error or warning is produced, and the CSS classes from those templates are missing from the compiled output.

This affects Adobe Commerce Cloud projects, which use the magento-cloud .gitignore. That file starts with * and selectively allows only application code and config, leaving vendor/ invisible to git and Tailwind.

The behavior is inconsistent depending on which gitignore style is used:

• Allow-list (* first, then ! exceptions): @source pointing to vendor/ fails silently. No classes are generated.
• Deny-list (vendor/ explicitly listed as ignored, as in the Magento 2 .gitignore): @source pointing to vendor/ works correctly. Classes are generated as expected.

The same @source directive, the same vendor/ directory on disk, two different outcomes based solely on how the .gitignore is structured.

Steps to reproduce

1. Create a .gitignore with an allow-list pattern:

   *
   !/app
   !/app/design/**
   

2. Place a template with Tailwind classes inside vendor/ (git-ignored):

   <!-- vendor/acme/theme/module/templates/component.phtml -->
   <div class="bg-blue-500 text-blue-50">This should be blue</div>

3. Reference the vendor directory via @source in your CSS entry point:

   @import "tailwindcss" source(none);
   @source "../../**/*.phtml";
   @source "../../**/*.xml";
   @source "../../../../../../vendor/acme/theme";

4. Run the Tailwind build.

Expected behavior

bg-blue-500 and text-blue-50 are present in the compiled CSS, because the path is explicitly declared via @source.

Actual behavior

bg-blue-500 and text-blue-50 are absent from the compiled CSS. Tailwind does not scan the vendor/ directory because git considers it non-existent (due to the allow-list gitignore). No warning or error is emitted.

Only classes from git-tracked files (e.g., bg-slate-500 from a file under app/design/) appear in the output.

[GrimLink]

Note this issue was made from #15452