fix(core): preserve nested-rule selectors in composition CSS scoping (#2721) (#2733)

Chrome 112+ / Firefox 117+ / Safari 16.5+ support native CSS Nesting.
A nested rule like '.title { … }' inside '[data-composition-id="intro"]
{ … }' resolves at match time to '<parent> .title' via the implicit
'&' prefix.

'scopeCssToComposition' walks every rule via 'root.walkRules' and re-
scopes selectors, but it did so for nested rules too — producing
'[…scope…] .title' inside '[…scope…] { … }', which nesting then
prepends AGAIN to '[…scope…] […scope…] .title'. Since the composition
root only appears once in the DOM, the doubly-scoped selector never
matches — the nested rule appears 'just ignored' as the reporter
described (#2721).

Reproduced on 0.7.66 with the reporter's exact composition. Fix: add
'isNestedInsideAnotherRule' predicate — mirrors the existing
'isInsideGlobalAtRule' — and skip nested rules in the walkRules
callback. Top-level rules still get scoped; their nested descendants
inherit scope naturally via CSS Nesting at match time.

Added two focused tests:
- 'preserves nested-rule selectors so CSS Nesting inheritance works
  (#2721)' — asserts nested '.title' and 'h2' selectors stay verbatim
  while top-level rules keep scoping.
- 'preserves deeply-nested CSS Nesting rules (#2721)' — same rule at
  depth 3.

All 37 existing scopeCssToComposition tests still pass.

Fixes #2721.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/core/src/compiler/compositionScoping.test.ts

@@ -800,6 +800,51 @@ window.__afterTimeline = window.__timelines.scene;
     expect(result).toContain('[id="intro"]');
   });
 
+  it("preserves nested-rule selectors so CSS Nesting inheritance works (#2721)", () => {
+    // Chrome 112+ / Firefox 117+ / Safari 16.5+ support native CSS Nesting.
+    // A nested rule like `.title { … }` inside `[data-composition-id="intro"] { … }`
+    // resolves at match time to `<parent> .title` via the implicit `&` prefix.
+    // The scoper must NOT re-apply the composition scope to the nested selector —
+    // that produces `<scope> <scope> .title`, which matches nothing because the
+    // composition root only appears once in the DOM.
+    const scoped = scopeCssToComposition(
+      `
+        [data-composition-id="intro"] h1 { color: red; }
+        [data-composition-id="intro"] {
+          .title { color: brown; }
+          h2 { color: blue; }
+        }
+      `,
+      "intro",
+    );
+    // Top-level rules still get scoped.
+    expect(scoped).toContain('[data-composition-id="intro"] h1');
+    // Nested rules keep their author-original selectors verbatim so CSS Nesting
+    // can prepend the parent's `&` at match time.
+    expect(scoped).toMatch(/\.title\s*\{/);
+    expect(scoped).toMatch(/h2\s*\{/);
+    // The pre-fix bug re-scoped nested rules to `[…] .title`, which never matched.
+    expect(scoped).not.toContain('[data-composition-id="intro"] .title');
+    expect(scoped).not.toContain('[data-composition-id="intro"] h2');
+  });
+
+  it("preserves deeply-nested CSS Nesting rules (#2721)", () => {
+    const scoped = scopeCssToComposition(
+      `
+        [data-composition-id="intro"] {
+          .card {
+            .header { font-weight: bold; }
+          }
+        }
+      `,
+      "intro",
+    );
+    expect(scoped).toMatch(/\.card\s*\{/);
+    expect(scoped).toMatch(/\.header\s*\{/);
+    expect(scoped).not.toContain('[data-composition-id="intro"] .card');
+    expect(scoped).not.toContain('[data-composition-id="intro"] .header');
+  });
+
   it("wraps scripts with authored root id normalization for #id GSAP selectors", () => {
     const { document } = parseHTML(`
       <div data-composition-id="intro">

packages/core/src/compiler/compositionScoping.ts

@@ -200,6 +200,24 @@ function isInsideGlobalAtRule(rule: Rule): boolean {
   return false;
 }
 
+/**
+ * A Rule nested inside another Rule (CSS Nesting Module Level 1) already
+ * inherits scope from its parent's `&` prefix at match time — re-applying
+ * the composition scope to the nested selector produces
+ * `<scope> <scope> .child`, which matches nothing when the composition
+ * root only appears once in the DOM. Only top-level rules get scoped;
+ * their nested descendants inherit the scope naturally via CSS nesting.
+ * See #2721 for the reproducer that motivated this.
+ */
+function isNestedInsideAnotherRule(rule: Rule): boolean {
+  let current: Node["parent"] = rule.parent;
+  while (current) {
+    if (current.type === "rule") return true;
+    current = current.parent;
+  }
+  return false;
+}
+
 export function scopeCssToComposition(
   css: string,
   compositionId: string,
@@ -216,6 +234,7 @@ export function scopeCssToComposition(
 
   root.walkRules((rule) => {
     if (isInsideGlobalAtRule(rule)) return;
+    if (isNestedInsideAnotherRule(rule)) return;
     rule.selectors = rule.selectors.map((selector) =>
       scopeSelector(
         selector,