From 7430d346394d3b608e4451eeee4105c0c9fb69fb Mon Sep 17 00:00:00 2001
From: Stephanie Zhang <stephanie.zhang@microsoft.com>
Date: Tue, 21 Jul 2026 09:18:46 -0700
Subject: [PATCH] [css-mixins] Make @apply and @contents rules StyleRuleGroups

@apply and @contents carry child rules but extended StyleRuleBase,
stashing them on a throwaway "fake parent" StyleRule. This CL makes them
extend StyleRuleGroup, owning their children directly and joining
DowncastTraits<StyleRuleGroup> and continuing the work from CL:7755161.

This is a behavior-preserving refactor. A web test was added confirming
that an empty `@apply --foo {}` block suppresses the mixin's @contents
fallback, while `@apply --foo;` still lets it apply.

Change-Id: I033b0d0b0e6d399152cb6ce3e1ac4dfd0c563c48
Fixed: 502048295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8114919
Commit-Queue: Stephanie Zhang <stephanie.zhang@microsoft.com>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1665525}
---
 css/css-mixins/mixins/contents-rule.html | 41 ++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/css/css-mixins/mixins/contents-rule.html b/css/css-mixins/mixins/contents-rule.html
index 0e7c4afc767b11..8d84186ae5eb82 100644
--- a/css/css-mixins/mixins/contents-rule.html
+++ b/css/css-mixins/mixins/contents-rule.html
@@ -130,5 +130,46 @@
         assert_equals(getComputedStyle(target).color, 'rgb(0, 128, 0)');
       }, 'Empty @contents parameter does not crash');
     </script>
+
+
+    <style>
+      @mixin --m8() {
+        @result {
+          @contents {
+            color: green;
+          }
+        }
+      }
+      #e8 {
+        @apply --m8;
+      }
+    </style>
+    <div id="e8">This text should be green.</div>
+    <script>
+      test(() => {
+        assert_equals(getComputedStyle(document.getElementById('e8')).color, 'rgb(0, 128, 0)');
+      }, '@apply with no block uses the @contents fallback');
+    </script>
+
+
+    <style>
+      @mixin --m9() {
+        @result {
+          @contents {
+            color: red;
+          }
+        }
+      }
+      #e9 {
+        color: green;
+        @apply --m9 {};
+      }
+    </style>
+    <div id="e9">This text should be green.</div>
+    <script>
+      test(() => {
+        assert_equals(getComputedStyle(document.getElementById('e9')).color, 'rgb(0, 128, 0)');
+      }, 'Empty @apply block suppresses the @contents fallback');
+    </script>
   </body>
 </html>