fix(css): scope the subject compound so theme-dark rules keep matching

scripts/scope-css.mjs

@@ -59,9 +59,26 @@ function scopeSelector(selector) {
   if (mentions(selector, ROOT)) {
     return selector;
   }
-  // `*` has to open a compound, and :is() takes that place now.
+  // The scope goes on the subject — the last compound — not the first:
-  const rest = selector[0]?.type === "universal" ? selector.slice(1) : selector;
+  // `.theme-dark .hue-chip` must keep matching with `.theme-dark` on body,
-  return [SCOPE, ...rest];
+  // outside the root.
+  let at = 0;
+  for (let i = 0; i < selector.length; i += 1) {
+    if (selector[i].type === "combinator") {
+      at = i + 1;
+    }
+  }
+  const head = selector.slice(0, at);
+  const subject = selector.slice(at);
+  // `*` and type selectors must open a compound; :is() goes in their place
+  // or right after them.
+  if (subject[0]?.type === "universal") {
+    return [...head, SCOPE, ...subject.slice(1)];
+  }
+  if (subject[0]?.type === "type") {
+    return [...head, subject[0], SCOPE, ...subject.slice(1)];
+  }
+  return [...head, SCOPE, ...subject];
 }
 
 /** Rewrites a built stylesheet so its rules only reach the plugin's own DOM. */

tests/scope-css.test.ts

@@ -48,6 +48,18 @@ describe("scoping the built stylesheet", () => {
     );
   });
 
+  it("scopes the subject, so an ancestor outside the root still matches", () => {
+    expect(one(".theme-dark .hue-chip{c:1}")).toBe(
+      ".theme-dark :is(.beaver-root, .beaver-root *).hue-chip { c: 1; }"
+    );
+    expect(one(".a > button{c:2}")).toBe(
+      ".a > button:is(.beaver-root, .beaver-root *) { c: 2; }"
+    );
+    expect(one(".a *{c:3}")).toBe(
+      ".a :is(.beaver-root, .beaver-root *) { c: 3; }"
+    );
+  });
+
   it("leaves rules that already name the root alone", () => {
     const reset = ".beaver-root.beaver-root button{cursor:pointer}";
     expect(one(reset)).toBe(