refactor: remove exact cross-layer css duplicates

tests/e2e/app-shell.spec.js

@@ -1803,6 +1803,30 @@ test("heart breathing prepares once then contracts on each exhale", async ({ pag
   await expect(page.locator(".heart-breath-ripple span").first()).toHaveCSS("transition-duration", "4s");
 });
 
+test("stylesheet layers do not repeat identical top-level rules", async ({ page }) => {
+  await mockApplication(page, session("admin", true));
+  await page.goto("/index.html");
+
+  const duplicates = await page.evaluate(() => {
+    const occurrences = new Map();
+    for (const sheet of Array.from(document.styleSheets)) {
+      const href = sheet.href ? new URL(sheet.href).pathname : "inline";
+      for (const rule of Array.from(sheet.cssRules || [])) {
+        if (typeof rule.selectorText !== "string" || !rule.style) continue;
+        const key = `${rule.selectorText}\u0000${rule.style.cssText}`;
+        const rows = occurrences.get(key) || [];
+        rows.push(href);
+        occurrences.set(key, rows);
+      }
+    }
+    return Array.from(occurrences.entries())
+      .filter(([, paths]) => new Set(paths).size > 1)
+      .map(([rule, paths]) => ({ rule, paths }));
+  });
+
+  expect(duplicates).toEqual([]);
+});
+
 test("mobile shell stays within the viewport", async ({ page }) => {
   await page.setViewportSize({ width: 375, height: 812 });
   await mockApplication(page, session("user", true));

tests/preservation_helpers.py

@@ -29,6 +29,34 @@ RETIRED_FRONTEND_SOURCE_RANGES = (
 )
 AUDITED_FRONTEND_SOURCE_LINE_COUNT = 9283
 
+# CR-12 removes only declarations that are repeated verbatim by a later
+# stylesheet layer under the same top-level cascade context. Keeping the exact
+# source fragments here makes every other byte of the accepted CSS baseline a
+# preservation requirement.
+AUDITED_CSS_RETIREMENTS = {
+    "styles.css": (
+        ".workspace-view.active-view {\n  display: block;\n}\n\n",
+        "body.sidebar-collapsed {\n  grid-template-columns: 64px minmax(0, 1fr);\n}\n\n",
+        ".rotation-day:last-child { border-right: 0; }\n\n",
+        (
+            "#screenerView .probability-value strong,\n"
+            "#screenerView .probability-value small {\n"
+            "  display: block;\n"
+            "}\n\n"
+        ),
+    ),
+    "renovation.css": (
+        ".overview-strip .sentiment-block { padding-left: 0; }\n",
+        (
+            '.overview-strip[data-overview-expanded="true"] .metric-value '
+            "{ font-size: 18px; }\n\n"
+        ),
+    ),
+    "redesign-v2.css": (
+        "#dragonView .dragon-operation-table .dragon-col-reason { width: auto; }\n",
+    ),
+}
+
 
 def sha256(path: Path) -> str:
     return hashlib.sha256(path.read_bytes()).hexdigest()
@@ -123,6 +151,17 @@ def assert_moved_asset_matches(
     frontend_relative: str | None = None,
 ) -> None:
     target_relative = frontend_relative or original_relative
+    if original_relative in AUDITED_CSS_RETIREMENTS:
+        original = (ORIGINAL_STATIC / original_relative).read_text(encoding="utf-8")
+        for retired in AUDITED_CSS_RETIREMENTS[original_relative]:
+            testcase.assertEqual(original.count(retired), 1, retired)
+            original = original.replace(retired, "", 1)
+        testcase.assertEqual(
+            (FRONTEND_ROOT / target_relative).read_text(encoding="utf-8"),
+            original,
+            original_relative,
+        )
+        return
     testcase.assertEqual(
         sha256(FRONTEND_ROOT / target_relative),
         sha256(ORIGINAL_STATIC / original_relative),

tests/test_preservation_slice_frontend.py

@@ -44,7 +44,7 @@ class FrontendPreservationSliceTests(unittest.TestCase):
             (ORIGINAL_STATIC / "index.html").read_text(encoding="utf-8"),
         )
 
-    def test_complete_stylesheet_stack_is_byte_identical_after_relocation(self) -> None:
+    def test_stylesheet_stack_matches_baseline_after_audited_retirements(self) -> None:
         for original, migrated in (
             ("shared/tokens.css", "shared/tokens.css"),
             ("styles.css", "styles/styles.css"),