fix(cssom): invalidate nested declaration mutations

moli-renderer-v8/src/live_stylesheet/mutation.rs

@@ -912,7 +912,7 @@ impl LiveStylesheet {
             return None;
         };
         let ancestor_path_len = rule_path.len().saturating_sub(1);
-        Some(LiveStylesheetRuleMutation::new(
+        Some(Self::rule_mutation_for_cascade(
             rule,
             self.css_rule_ancestors_at_path(&rule_path[..ancestor_path_len])?,
             change_kind,
@@ -925,13 +925,36 @@ impl LiveStylesheet {
         parent_path: &[usize],
         change_kind: RuleChangeKind,
     ) -> Option<LiveStylesheetRuleMutation> {
-        Some(LiveStylesheetRuleMutation::new(
+        Some(Self::rule_mutation_for_cascade(
             rule,
             self.css_rule_ancestors_at_path(parent_path)?,
             change_kind,
         ))
     }
 
+    fn rule_mutation_for_cascade(
+        rule: CssRule,
+        ancestors: Vec<CssRule>,
+        change_kind: RuleChangeKind,
+    ) -> LiveStylesheetRuleMutation {
+        if matches!(&rule, CssRule::NestedDeclarations(_))
+            && let Some(style_index) = ancestors
+                .iter()
+                .rposition(|ancestor| matches!(ancestor, CssRule::Style(_)))
+        {
+            // Nested declarations inherit their selector from the nearest
+            // enclosing style rule. Stylo therefore does not collect ordinary
+            // element invalidations from the declaration rule itself. Report
+            // the effective declaration change on that selector-bearing rule.
+            return LiveStylesheetRuleMutation::new(
+                ancestors[style_index].clone(),
+                ancestors[..style_index].to_vec(),
+                RuleChangeKind::StyleRuleDeclarations,
+            );
+        }
+        LiveStylesheetRuleMutation::new(rule, ancestors, change_kind)
+    }
+
     fn css_rule_ancestors_at_path(&self, path: &[usize]) -> Option<Vec<CssRule>> {
         let mut ancestors = Vec::with_capacity(path.len());
         for depth in 1..=path.len() {

moli-renderer-v8/src/script_vm/tests/dom_xhr/computed_style.rs

@@ -3254,6 +3254,73 @@ getComputedStyle(document.querySelector('.nested-journal-target')).color;
     );
 }
 
+#[test]
+fn nested_declaration_mutations_invalidate_their_ancestor_style_selector() {
+    let mut vm = new_storage_test_vm("https://nested-declaration-invalidation.test/");
+
+    assert_eq!(
+        vm.eval(
+            r#"
+const style = document.createElement('style');
+style.textContent = 'div { z-index: 1; &.test { } }';
+(document.head || document.documentElement || document).appendChild(style);
+const target = document.createElement('div');
+target.className = 'test';
+(document.body || document.documentElement || document).appendChild(target);
+globalThis.__nestedDeclarationRule = style.sheet.cssRules[0];
+globalThis.__nestedDeclarationTarget = target;
+getComputedStyle(target).zIndex;
+"#,
+        )
+        .expect("nested declaration invalidation setup should evaluate"),
+        "1"
+    );
+    crate::style_engine::reset_live_stylesheet_update_counts_for_test();
+
+    assert_eq!(
+        vm.eval(
+            r#"
+const rule = globalThis.__nestedDeclarationRule;
+const mutationTarget = globalThis.__nestedDeclarationTarget;
+const states = [];
+
+rule.insertRule('z-index: 3;', 0);
+const declarations = rule.cssRules[0];
+states.push(declarations instanceof CSSNestedDeclarations);
+states.push(getComputedStyle(mutationTarget).zIndex);
+
+declarations.style.zIndex = '4';
+states.push(getComputedStyle(mutationTarget).zIndex);
+
+rule.deleteRule(0);
+states.push(getComputedStyle(mutationTarget).zIndex);
+
+rule.insertRule('@media all { a { } }', 1);
+const media = rule.cssRules[1];
+media.insertRule('z-index: 5;', 0);
+states.push(media.cssRules[0] instanceof CSSNestedDeclarations);
+states.push(getComputedStyle(mutationTarget).zIndex);
+
+media.deleteRule(0);
+states.push(getComputedStyle(mutationTarget).zIndex);
+states.join('|');
+"#,
+        )
+        .expect("nested declaration mutations should invalidate ancestor selectors"),
+        "true|3|4|1|true|5|1"
+    );
+    assert_eq!(
+        crate::style_engine::exact_rule_change_notification_count_for_test(),
+        6,
+        "nested declaration changes should remain exact journal updates",
+    );
+    assert_eq!(
+        crate::style_engine::full_cascade_update_fallback_count_for_test(),
+        0,
+        "nested declaration changes must not rebuild an entire stylesheet",
+    );
+}
+
 #[test]
 fn computed_style_wrapper_reflects_style_element_media_mutations() {
     let mut vm = new_storage_test_vm("https://style-media-computed-wrapper-refresh.test/");