fix(ui): merge nav + sticky filter into a single backdrop-filter band

Two adjacent backdrop-filter elements (nav at y=0-80, filter at y=80+)
always show a visible seam at their boundary because each filter clips
its own blur kernel, so the edge pixels sample slightly different
neighborhoods. Same recipe doesn't help — it's a structural issue.

Fix: when filter is stuck, render an absolutely-positioned glass child
inside the filter that extends from -top-20 to bottom-0 (i.e. covers
nav area + filter area as ONE element). Nav reads filterStuck from a
tiny shared zustand UI store and disables its own glass layer in that
state, so only the shared band is visible. Single element, single
backdrop-filter, no seam.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/page.tsx

@@ -11,6 +11,7 @@ import { getActivityEndTime, sortArtists, type SortKey } from "@/lib/mock-data";
 import { useVoteStore } from "@/lib/store";
 import { useVoteAction } from "@/hooks/useVoteAction";
 import { useScrollRestore } from "@/hooks/useScrollRestore";
+import { useUIStore } from "@/lib/ui-store";
 import { cn } from "@/lib/cn";
 import { tosUrl } from "@/lib/tos";
 
@@ -23,6 +24,7 @@ export default function Home() {
   const [sortKey, setSortKey] = useState<SortKey>("votes");
   const [filterStuck, setFilterStuck] = useState(false);
   const filterSentinelRef = useRef<HTMLDivElement>(null);
+  const setStoreFilterStuck = useUIStore((s) => s.setFilterStuck);
 
   const endTime = useMemo(() => getActivityEndTime(), []);
 
@@ -66,6 +68,13 @@ export default function Home() {
     };
   }, []);
 
+  // 把 filterStuck 同步到全局 UI store —— 让导航栏感知,在吸顶时关掉自己的玻璃,
+  // 让筛选条延伸出的"共享玻璃带"成为唯一的 backdrop-filter,消除接缝
+  useEffect(() => {
+    setStoreFilterStuck(filterStuck);
+    return () => setStoreFilterStuck(false);
+  }, [filterStuck, setStoreFilterStuck]);
+
   return (
     <>
       {/* Hero · 全屏沉浸式视频 · 作为第一个 snap 点
@@ -119,16 +128,25 @@ export default function Home() {
         {/* 哨兵：用于检测筛选条是否吸顶 */}
         <div ref={filterSentinelRef} aria-hidden style={{ height: 0 }} />
 
-        {/* 筛选条 · 外层铺满（与导航栏同宽），吸顶后启用毛玻璃；内层版心承载文案 */}
+        {/* 筛选条 · 外层铺满,内层版心承载文案。
+            吸顶时,absolute 子层把玻璃从 -top-20 一路扩到容器底部 ——
+            这是一个单一元素的 backdrop-filter,横跨 nav 区域 + filter 区域,
+            消除两块独立玻璃在 y=80 接缝处的视觉割裂。导航栏同步关掉自己的玻璃。 */}
         <div
-          className={cn(
-            "sticky z-30 transition-colors duration-200",
-            filterStuck &&
-              "bg-surface/40 backdrop-blur-xl backdrop-saturate-150 border-b border-white/[0.06]",
-          )}
+          className="sticky z-30 transition-colors duration-200"
           style={{ top: "80px" }}
         >
-          <div className="max-w-[1500px] mx-auto px-4 sm:px-6 lg:px-8">
+          {/* 共享玻璃带:absolute,吸顶时延伸到 nav 顶部,opacity 平滑过渡 */}
+          <div
+            aria-hidden
+            className={cn(
+              "absolute inset-x-0 -top-20 bottom-0 pointer-events-none",
+              "bg-surface/40 backdrop-blur-xl backdrop-saturate-150 border-b border-white/[0.06]",
+              "transition-opacity duration-300",
+              filterStuck ? "opacity-100" : "opacity-0",
+            )}
+          />
+          <div className="relative max-w-[1500px] mx-auto px-4 sm:px-6 lg:px-8">
             <ArtistFilters
               tagFilter={tagFilter}
               onTagChange={setTagFilter}

src/components/Navigation.tsx

@@ -8,6 +8,7 @@ import SearchTrigger from "./SearchTrigger";
 import AuthMenu from "./auth/AuthMenu";
 import RemainingVotesBadge from "./auth/RemainingVotesBadge";
 import { cn } from "@/lib/cn";
+import { useUIStore } from "@/lib/ui-store";
 
 /**
  * 导航栏 · 上下文敏感的玻璃态切换
@@ -23,6 +24,10 @@ export default function Navigation() {
   const pathname = usePathname();
   // 初始乐观:任何页面首屏都假设在顶部/Hero 上,透明。effect 挂载后会立刻校正。
   const [isTransparent, setIsTransparent] = useState(true);
+  // 筛选条吸顶时,nav 也变透明 —— 此时筛选条的"共享玻璃带"已延伸到 nav 顶部,
+  // nav 关掉自己的玻璃,避免双重 backdrop-filter 在 y=80 处出现拼接线。
+  const filterStuck = useUIStore((s) => s.filterStuck);
+  const glassOff = isTransparent || filterStuck;
 
   // 维护一个站内导航计数器(per-tab),供 FloatingBackButton 判断 router.back() 是否安全
   useEffect(() => {
@@ -57,12 +62,15 @@ export default function Navigation() {
 
   return (
     <header className="fixed top-0 inset-x-0 z-50">
-      {/* 玻璃层 · 通过 opacity 渐显渐隐 · bg-surface 与全站卡片同色 */}
+      {/* 玻璃层 · 通过 opacity 渐显渐隐 · bg-surface 与全站卡片同色
+          glassOff = isTransparent || filterStuck:
+          - isTransparent: Hero / 页顶,本来就要透明
+          - filterStuck: 筛选条已伸出共享玻璃,nav 关掉自己的玻璃避免接缝 */}
       <div
         aria-hidden
         className={cn(
           "absolute inset-0 bg-surface/40 backdrop-blur-xl backdrop-saturate-150 transition-opacity duration-300",
-          isTransparent ? "opacity-0" : "opacity-100",
+          glassOff ? "opacity-0" : "opacity-100",
         )}
       />
       {/* 顶部 1px 高光 · 仅玻璃态下显示 */}
@@ -70,7 +78,7 @@ export default function Navigation() {
         aria-hidden
         className={cn(
           "absolute inset-x-0 top-0 h-px bg-white/[0.06] transition-opacity duration-300",
-          isTransparent ? "opacity-0" : "opacity-100",
+          glassOff ? "opacity-0" : "opacity-100",
         )}
       />
       <nav className="relative max-w-[1500px] mx-auto h-20 px-4 sm:px-6 lg:px-8 flex items-center gap-8">
@@ -91,7 +99,7 @@ export default function Navigation() {
       <NavLinks
         className={cn(
           "relative md:hidden overflow-x-auto transition-colors duration-300",
-          isTransparent
+          glassOff
             ? "border-t border-transparent"
             : "border-t border-white/[0.05]",
         )}

src/lib/ui-store.ts

@@ -0,0 +1,18 @@
+import { create } from "zustand";
+
+/**
+ * 跨组件 UI 协调状态。
+ * 主要用途:首页筛选条吸顶时通知导航关掉自己的玻璃,
+ * 让筛选条把玻璃从 y=0 一路延伸到自己底部,形成单一连续 backdrop-filter,
+ * 消除两块独立玻璃在 y=80 处的接缝。
+ */
+interface UIStore {
+  /** 当前页面有 sticky 筛选条且已经吸顶 */
+  filterStuck: boolean;
+  setFilterStuck: (stuck: boolean) => void;
+}
+
+export const useUIStore = create<UIStore>((set) => ({
+  filterStuck: false,
+  setFilterStuck: (stuck) => set({ filterStuck: stuck }),
+}));