improve source map generation in dev mode, fixes #4790

Closes #4790

Merge request studip/studip!3583

webpack.dev.js

@@ -11,7 +11,7 @@ const statusesPaths = {
 
 module.exports = merge(common, {
     mode: 'development',
-    devtool: 'eval-cheap-module-source-map',
+    devtool: false,
     plugins: [
         new webpack.WatchIgnorePlugin({
             paths:[
@@ -28,5 +28,10 @@ module.exports = merge(common, {
             excludeWarnings: true,
             contentImage: statusesPaths,
         }),
+        new webpack.SourceMapDevToolPlugin({
+            test: /\.css$/i,
+            filename: null,
+            append: '/*# sourceMappingURL=[url] */',
+        }),
     ],
 });