Fix Next.js endless loop when setting a custom distDir (#15053)

Closes #15050

In Tailwind CSS v4 Alpha 31 we changed how we scan template files. This
changes included a new folder-dependency that is emitted for the `base`
directory, so we can listen for new files being added as part of the
postcss dependency.

In our testing, this worked fine with the Next.js integration meaning a
new file in the project root would be picked up by Oxide and we could
update the CSS files accordingly.

This change is now, however, causing an issue. With Next.js 15 **and
with a custom `distDir` configured**, the postcss build, that will write
into the `distDir`, will cause another postcss run to be triggered,
starting an endless loop (regardless of wether or not the `distDir` was
also part of your gitignore list).

This PR now changes the postcss client to not emit the base directory as
a dependency to revert this changes. This does mean that new files and
folders created _directly in the project root_ will require a restart of
the Next.js server again (just like it did in Alpha 31 and before) for
now.

## Test Plan

Next 15 does not seem to run in our current integration test setup (for
some reason the server does not close correctly and it will fail on the
cleanup step), so this change was tested manually:

- First, clone the [templates
repo](https://github.com/philipp-spiess/tailwindcss-playgrounds) I use
for third party frameworks
- Then, do a full build in the parent repo `tailwindcss` via `pnpm
build`
- Now, install the local tarballs in the `tailwindcss-playgrounds` repo
via `pnpm install`

With this setup I have tested changes to a template file (that causes
new utilities to be added) and the CSS file (that will rebuild properly)
across both `pnpm dev` and `pnpm dev --turbo`. Furthermore integration
tests assert it still works in Next 14 like it did before:


https://github.com/user-attachments/assets/b0ccb3dd-d090-4e4c-97c5-74129a2789be

One thing to make sure of is to include the new `distDir` into the
`.gitignore` file as well, otherwise we will scrape it for changes which
inherently causes an endless loop issue again.

---------

Co-authored-by: Adam Wathan <[email protected]>

Author: philipp-spiess

packages/@tailwindcss-postcss/src/index.ts

@@ -170,19 +170,26 @@ function tailwindcss(opts: PluginOptions = {}): AcceptedPlugin {
           // Register dependencies so changes in `base` cause a rebuild while
           // giving tools like Vite or Parcel a glob that can be used to limit
           // the files that cause a rebuild to only those that match it.
-          for (let { base, pattern } of context.scanner.globs) {
+          for (let { base: globBase, pattern } of context.scanner.globs) {
+            // Avoid adding a dependency on the base directory itself, since it
+            // causes Next.js to start an endless recursion if the `distDir` is
+            // configured to anything other than the default `.next` dir.
+            if (pattern === '*' && base === globBase) {
+              continue
+            }
+
             if (pattern === '') {
               result.messages.push({
                 type: 'dependency',
                 plugin: '@tailwindcss/postcss',
-                file: base,
+                file: globBase,
                 parent: result.opts.from,
               })
             } else {
               result.messages.push({
                 type: 'dir-dependency',
                 plugin: '@tailwindcss/postcss',
-                dir: base,
+                dir: globBase,
                 glob: pattern,
                 parent: result.opts.from,
               })