Fix it so css.TailwindCSS inlineImports options isn't always enabled

To avoid breaking existing setup and to make a better default option, the option is now `disableInlineImports` (default false).
Fixes #13719

resources/resource_transformers/cssjs/postcss.go

@@ -69,7 +69,6 @@ func (c *PostCSSClient) Process(res resources.ResourceTransformer, options map[s
 }
 
 type InlineImports struct {
-	// Service `mapstructure:",squash"`
 	// Enable inlining of @import statements.
 	// Does so recursively, but currently once only per file;
 	// that is, it's not possible to import the same file in
@@ -78,6 +77,11 @@ type InlineImports struct {
 	// so you can have @import anywhere in the file.
 	InlineImports bool
 
+	// See issue https://github.com/gohugoio/hugo/issues/13719
+	// Disable inlining of @import statements
+	// This is currenty only used for css.TailwindCSS.
+	DisableInlineImports bool
+
 	// When InlineImports is enabled, we fail the build if an import cannot be resolved.
 	// You can enable this to allow the build to continue and leave the import statement in place.
 	// Note that the inline importer does not process url location or imports with media queries,

resources/resource_transformers/cssjs/tailwindcss.go

@@ -129,10 +129,12 @@ func (t *tailwindcssTransformation) Transform(ctx *resources.ResourceTransformat
 		t.rs.Assets.Fs, t.rs.Logger, ctx.DependencyManager,
 	)
 
+	if !options.InlineImports.DisableInlineImports {
 		src, err = imp.resolve()
 		if err != nil {
 			return err
 		}
+	}
 
 	go func() {
 		defer stdin.Close()
@@ -146,7 +148,11 @@ func (t *tailwindcssTransformation) Transform(ctx *resources.ResourceTransformat
 				Cause: err,
 			}
 		}
-		return imp.toFileError(errBuf.String())
+		s := errBuf.String()
+		if options.InlineImports.DisableInlineImports && strings.Contains(s, "Can't resolve") {
+			s += "You may want to set the 'disableInlineImports' option to false to inline imports, see https://gohugo.io/functions/css/tailwindcss/#disableinlineimports"
+		}
+		return imp.toFileError(s)
 	}
 
 	return nil

resources/resource_transformers/cssjs/tailwindcss_integration_test.go

@@ -17,6 +17,7 @@ import (
 	"testing"
 
 	"github.com/bep/logg"
+	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/htesting"
 	"github.com/gohugoio/hugo/hugolib"
 )
@@ -70,3 +71,66 @@ CSS: {{ $css.Content | safeCSS }}|
 
 	b.AssertFileContent("public/index.html", "/*! tailwindcss v4.")
 }
+
+func TestTailwindCSSNoInlineImportsIssue13719(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+theme = 'my-theme'
+
+[[module.mounts]]
+source = 'assets'
+target = 'assets'
+
+[[module.mounts]]
+source = 'other'
+target = 'assets/css'
+-- assets/css/main.css --
+@import "tailwindcss";
+
+@import "colors/red.css";
+@import "colors/blue.css";
+@import "colors/purple.css";
+-- assets/css/colors/red.css --
+@import "green.css";
+
+.red {color: red;}
+-- assets/css/colors/green.css --
+.green {color: green;}
+-- themes/my-theme/assets/css/colors/blue.css --
+.blue {color: blue;}
+-- other/colors/purple.css --
+.purple {color: purple;}
+-- layouts/home.html --
+{{ with (templates.Defer (dict "key" "global")) }}
+  {{ with resources.Get "css/main.css" }}
+    {{ $opts := dict "disableInlineImports" true }}
+    {{ with . | css.TailwindCSS $opts }}
+      <link rel="stylesheet" href="{{ .RelPermalink }}">
+    {{ end }}
+  {{ end }}
+{{ end }}
+-- package.json --
+{
+  "devDependencies": {
+    "@tailwindcss/cli": "^4.1.7",
+    "tailwindcss": "^4.1.7"
+  }
+}
+`
+
+	b, err := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:               t,
+			TxtarString:     files,
+			NeedsOsFS:       true,
+			NeedsNpmInstall: true,
+			LogLevel:        logg.LevelInfo,
+		}).BuildE()
+
+	b.Assert(err, qt.IsNotNil)
+	b.Assert(err.Error(), qt.Contains, "Can't resolve 'colors/red.css'")
+	b.Assert(err.Error(), qt.Contains, "You may want to set the 'disableInlineImports' option to false")
+}