vite: Fix .css.ts file resolution on Windows (#1346)

Author: askoufis

.changeset/eight-peas-provide.md

@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/vite-plugin': patch
+---
+
+Fix `.css.ts` file resolution on Windows

packages/vite-plugin/src/index.ts

@@ -47,8 +47,10 @@ export function vanillaExtractPlugin({
     if (
       filePath.startsWith(config.root) ||
       // In monorepos the absolute path will be outside of config.root, so we check that they have the same root on the file system
+      // Paths from vite are always normalized, so we have to use the posix path separator
       (path.isAbsolute(filePath) &&
-        filePath.split(path.sep)[1] === config.root.split(path.sep)[1])
+        filePath.split(path.posix.sep)[1] ===
+          config.root.split(path.posix.sep)[1])
     ) {
       resolvedId = filePath;
     } else {