fix: sourcemap path mangled by browser (#1326)

Closes #1323

packages/vite/src/node/server/middlewares/transform.ts

@@ -15,7 +15,12 @@ import { send } from '../send'
 import { transformRequest } from '../transformRequest'
 import { isHTMLProxy } from '../../plugins/html'
 import chalk from 'chalk'
-import { DEP_CACHE_DIR, DEP_VERSION_RE, VALID_ID_PREFIX } from '../../constants'
+import {
+  DEP_CACHE_DIR,
+  DEP_VERSION_RE,
+  FS_PREFIX,
+  VALID_ID_PREFIX
+} from '../../constants'
 
 const debugCache = createDebugger('vite:cache')
 const isDebug = !!process.env.DEBUG
@@ -46,6 +51,10 @@ export function transformMiddleware(
       const isSourceMap = withoutQuery.endsWith('.map')
       // since we generate source map references, handle those requests here
       if (isSourceMap) {
+        // #1323 - browser may remove // when fetching source maps
+        if (url.startsWith(FS_PREFIX)) {
+          url = FS_PREFIX + url.split(FS_PREFIX)[1].replace(/^\/?/, '/')
+        }
         const originalUrl = url.replace(/\.map($|\?)/, '$1')
         const map = (await moduleGraph.getModuleByUrl(originalUrl))
           ?.transformResult?.map