fix(compiler): prevent shimCssText from adding extra blank lines per CSS comment

mattlewis92 · leonsenft · commit 5a712d42d161 · 2026-03-20T15:17:31.000-07:00
The comment placeholder restoration in `shimCssText` appended an unconditional
`+ '\n'` to each non-hash comment replacement. Because `_commentRe` does not
consume the newline that follows a comment in the source, that newline already
remains in `cssText`. The extra `'\n'` was therefore inserted on top of the
existing one, shifting every line after each comment down by one. In files with
many comments (e.g. large SCSS preambles) this shifts all subsequent CSS rules
far enough that the CSS sourcemap &mdash; generated before `shimCssText` runs &mdash;
points to completely wrong source locations in browser DevTools.

The fix is to drop the `+ '\n'`; internal newlines within a multi-line comment
are still preserved via `_newLinesRe`, and the trailing newline that follows the
comment in `cssText` is already present without any extra injection.
diff --git a/packages/compiler/src/shadow_css.ts b/packages/compiler/src/shadow_css.ts
@@ -186,7 +186,7 @@ export class ShadowCss {
         // Replace non hash comments with empty lines.
         // This is done so that we do not leak any sensitive data in comments.
         const newLinesMatches = m.match(_newLinesRe);
-        comments.push((newLinesMatches?.join('') ?? '') + '\n');
+        comments.push(newLinesMatches?.join('') ?? '');
       }
 
       return COMMENT_PLACEHOLDER;
diff --git a/packages/compiler/test/shadow_css/shadow_css_spec.ts b/packages/compiler/test/shadow_css/shadow_css_spec.ts
@@ -368,17 +368,17 @@ describe('ShadowCss', () => {
   describe('comments', () => {
     // Comments should be kept in the same position as otherwise inline sourcemaps break due to
     // shift in lines.
-    it('should replace multiline comments with newline', () => {
-      expect(shim('/* b {c} */ b {c}', 'contenta')).toBe('\n b[contenta] {c}');
+    it('should remove inline comments without adding extra lines', () => {
+      expect(shim('/* b {c} */ b {c}', 'contenta')).toBe(' b[contenta] {c}');
     });
 
-    it('should replace multiline comments with newline in the original position', () => {
-      expect(shim('/* b {c}\n */ b {c}', 'contenta')).toBe('\n\n b[contenta] {c}');
+    it('should preserve internal newlines from multiline comments', () => {
+      expect(shim('/* b {c}\n */ b {c}', 'contenta')).toBe('\n b[contenta] {c}');
     });
 
-    it('should replace comments with newline in the original position', () => {
+    it('should remove multiple inline comments without adding extra lines', () => {
       expect(shim('/* b {c} */ b {c} /* a {c} */ a {c}', 'contenta')).toBe(
-        '\n b[contenta] {c} \n a[contenta] {c}',
+        ' b[contenta] {c}  a[contenta] {c}',
       );
     });
 
@@ -392,9 +392,7 @@ describe('ShadowCss', () => {
     });
 
     it('should handle adjacent comments', () => {
-      expect(shim('/* comment 1 */ /* comment 2 */ b {c}', 'contenta')).toBe(
-        '\n \n b[contenta] {c}',
-      );
+      expect(shim('/* comment 1 */ /* comment 2 */ b {c}', 'contenta')).toBe('  b[contenta] {c}');
     });
   });
 });

Original file line number	Diff line number	Diff line change
`@@ -186,7 +186,7 @@ export class ShadowCss {`
`186`	`186`	`// Replace non hash comments with empty lines.`
`187`	`187`	`// This is done so that we do not leak any sensitive data in comments.`
`188`	`188`	`const newLinesMatches = m.match(_newLinesRe);`
`189`		`- comments.push((newLinesMatches?.join('') ?? '') + '\n');`
	`189`	`+ comments.push(newLinesMatches?.join('') ?? '');`
`190`	`190`	`}`
`191`	`191`
`192`	`192`	`return COMMENT_PLACEHOLDER;`