diff --git a/frontend/app/admin/layout.tsx b/frontend/app/admin/layout.tsx
index b0d7e78..3bda987 100644
--- a/frontend/app/admin/layout.tsx
+++ b/frontend/app/admin/layout.tsx
@@ -28,7 +28,6 @@ export default async function AdminLayout({ children }: { children: React.ReactN
         id="main"
         tabIndex={-1}
         className="min-h-screen flex items-center justify-center px-4 outline-none"
-        style={{ background: "var(--bg)" }}
       >
         <div className="max-w-md w-full panel p-8 text-center">
           <span
diff --git a/frontend/app/announcement/[id]/page.tsx b/frontend/app/announcement/[id]/page.tsx
index 5af0375..8e151ca 100644
--- a/frontend/app/announcement/[id]/page.tsx
+++ b/frontend/app/announcement/[id]/page.tsx
@@ -103,6 +103,15 @@ export default async function AnnouncementPage({ params }: PageProps) {
       <SiteDocShell
         left={left}
         right={right}
+        breadcrumb={
+          <SiteDocBreadcrumb
+            items={[
+              { href: "/", label: "首页" },
+              { href: "/announcements", label: "站点公告" },
+              { label: a.title },
+            ]}
+          />
+        }
         mobileExtras={
           <>
             {toc.length > 0 ? <PostToc items={toc} /> : null}
@@ -111,14 +120,6 @@ export default async function AnnouncementPage({ params }: PageProps) {
           </>
         }
       >
-        <SiteDocBreadcrumb
-          items={[
-            { href: "/", label: "首页" },
-            { href: "/announcements", label: "站点公告" },
-            { label: a.title },
-          ]}
-        />
-
         <article className="panel p-6 sm:p-8">
           <SiteDocMasthead
             kicker="站点公告"
diff --git a/frontend/app/announcements/page.tsx b/frontend/app/announcements/page.tsx
index 5c641d8..a823a2f 100644
--- a/frontend/app/announcements/page.tsx
+++ b/frontend/app/announcements/page.tsx
@@ -132,15 +132,16 @@ export default async function AnnouncementsListPage({ searchParams }: PageProps)
     <SiteDocShell
       left={left}
       right={right}
+      breadcrumb={
+        <SiteDocBreadcrumb
+          items={[
+            { href: "/", label: "首页" },
+            { label: "站点公告" },
+          ]}
+        />
+      }
       mobileExtras={<SiteDocPageNav pages={sidebarPages} title="关于本站" />}
     >
-      <SiteDocBreadcrumb
-        items={[
-          { href: "/", label: "首页" },
-          { label: "站点公告" },
-        ]}
-      />
-
       <header className="mb-5">
         <h1
           className="text-[24px] sm:text-[28px] font-extrabold tracking-tight flex items-center gap-2"
diff --git a/frontend/app/compose/page.tsx b/frontend/app/compose/page.tsx
index 59be47b..7358369 100644
--- a/frontend/app/compose/page.tsx
+++ b/frontend/app/compose/page.tsx
@@ -42,7 +42,7 @@ function ComposeInner() {
 export default function ComposePage() {
   return (
     <div className="j13-compose-page">
-      <nav className="meta mb-4 flex items-center gap-1.5 text-[13px] shrink-0">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none shrink-0">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">
           首页
         </Link>
diff --git a/frontend/app/globals.css b/frontend/app/globals.css
index bc407c2..4d25dc6 100644
--- a/frontend/app/globals.css
+++ b/frontend/app/globals.css
@@ -56,6 +56,14 @@
   --r: 18px;
   --r-sm: 12px;
 
+  /* 顶栏 68px 条 + 1px 底边；main 桌面 py-9。侧栏 sticky 必须等于
+     栏在 scrollY=0 时的视口 top，否则第一下滚动会先空走一截再吸住 */
+  --header-bar: 68px;
+  --header-h: calc(var(--header-bar) + 1px);
+  --main-pad-y: 2.25rem;
+  --rail-top: calc(var(--header-h) + var(--main-pad-y));
+  --rail-max-h: calc(100vh - var(--rail-top) - 1rem);
+
   color-scheme: light;
 }
 
@@ -103,6 +111,7 @@
 
 html {
   scrollbar-color: var(--line-2) transparent;
+  background-color: var(--bg); /* 橡皮筋 overscroll 与画布同色 */
 }
 
 body {
@@ -113,6 +122,59 @@ body {
   -webkit-font-smoothing: auto;
   text-rendering: optimizeLegibility;
   font-synthesis: none; /* 禁止伪粗，避免缺档字重糊成一团 */
+  isolation: isolate; /* 让 z-index:-1 的画布层停在内容之下、html 之上 */
+}
+
+/* 浏览器最后一层画布：氛围光 + 石墨微粒。
+   纯 CSS（令牌 + 内联 SVG），不跟位图——跟浅/暗/accent 换肤，无限缩放，无额外请求。 */
+body::before {
+  content: "";
+  position: fixed;
+  inset: 0;
+  z-index: -1;
+  pointer-events: none;
+  background-image:
+    radial-gradient(ellipse 80% 52% at 12% -18%, color-mix(in srgb, var(--accent) 10%, transparent), transparent 62%),
+    radial-gradient(ellipse 48% 36% at 100% 0%, color-mix(in srgb, var(--ink) 4%, transparent), transparent 58%),
+    radial-gradient(ellipse 42% 28% at 78% 100%, color-mix(in srgb, var(--accent) 6%, transparent), transparent 60%),
+    linear-gradient(180deg, color-mix(in srgb, var(--panel) 40%, transparent) 0%, transparent 32%);
+}
+
+body::after {
+  content: "";
+  position: fixed;
+  inset: 0;
+  z-index: -1;
+  pointer-events: none;
+  opacity: 0.032;
+  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 160 160' width='160' height='160'%3E%3Cfilter id='n' x='0' y='0' width='100%25' height='100%25'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='.8' numOctaves='4' stitchTiles='stitch'/%3E%3CfeColorMatrix type='saturate' values='0'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
+  background-size: 160px 160px;
+}
+
+.dark body::before {
+  background-image:
+    radial-gradient(ellipse 80% 52% at 12% -18%, color-mix(in srgb, var(--accent) 16%, transparent), transparent 62%),
+    radial-gradient(ellipse 48% 36% at 100% 0%, color-mix(in srgb, var(--ink) 6%, transparent), transparent 58%),
+    radial-gradient(ellipse 42% 28% at 78% 100%, color-mix(in srgb, var(--accent) 9%, transparent), transparent 60%),
+    linear-gradient(180deg, color-mix(in srgb, var(--panel) 22%, transparent) 0%, transparent 36%);
+}
+
+.dark body::after {
+  opacity: 0.05;
+}
+
+@media print {
+  body::before,
+  body::after {
+    display: none;
+  }
+}
+
+@media (prefers-contrast: more) {
+  body::before,
+  body::after {
+    display: none;
+  }
 }
 
 /* 全站悬停即显提示气泡（配合 TipHost + data-tip，替代原生 title 延迟） */
@@ -2319,7 +2381,6 @@ body:has([data-admin-viewport]) {
   height: 100dvh;
   max-height: 100dvh;
   overflow: hidden;
-  background: var(--bg);
 }
 
 .admin-shell {
@@ -2328,7 +2389,7 @@ body:has([data-admin-viewport]) {
   max-height: 100dvh;
   min-height: 0;
   overflow: hidden;
-  background: var(--bg);
+  background: transparent; /* 透出全站画布层 */
 }
 
 .admin-rail {
@@ -5136,6 +5197,58 @@ html.dark .prose-content pre[data-theme] span {
   font-size: 13px;
 }
 
+/* 长文回顶：右下角浮钮 */
+.j13-back-top {
+  position: fixed;
+  right: 1rem;
+  bottom: 1.25rem;
+  z-index: 45;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 2.75rem;
+  height: 2.75rem;
+  border-radius: 999px;
+  border: 1px solid var(--line-2);
+  background: color-mix(in srgb, var(--panel) 92%, transparent);
+  color: var(--ink-2);
+  box-shadow: var(--shadow-lg);
+  backdrop-filter: blur(14px);
+  -webkit-backdrop-filter: blur(14px);
+  cursor: pointer;
+  transition: color 0.15s ease, background 0.15s ease, border-color 0.15s ease;
+}
+.j13-back-top:hover,
+.j13-back-top:focus-visible {
+  color: var(--accent);
+  background: var(--accent-soft);
+  border-color: color-mix(in srgb, var(--accent) 28%, var(--line-2));
+  outline: none;
+}
+/* 帖子移动端底栏上方抬高，避免遮挡 */
+.j13-back-top.is-lifted {
+  bottom: 5.25rem;
+}
+@media (min-width: 1024px) {
+  .j13-back-top.is-lifted {
+    bottom: 1.25rem;
+  }
+}
+@media (min-width: 640px) {
+  .j13-back-top {
+    right: 1.35rem;
+    bottom: 1.5rem;
+  }
+  .j13-back-top.is-lifted {
+    bottom: 5.25rem;
+  }
+}
+@media (min-width: 1024px) {
+  .j13-back-top.is-lifted {
+    bottom: 1.5rem;
+  }
+}
+
 /* 评论楼层树：抖音风格
    主评论全宽；子回复折叠在 panel-2 背景容器内，点击"展开 X 条回复"显示
    子回复之间用淡分隔线；depth≥2 时昵称后显示 ▶ 被回复者 */
diff --git a/frontend/app/notifications/page.tsx b/frontend/app/notifications/page.tsx
index 4fdd2fe..b0d146f 100644
--- a/frontend/app/notifications/page.tsx
+++ b/frontend/app/notifications/page.tsx
@@ -32,7 +32,7 @@ export default async function NotificationsPage({ searchParams }: NotificationsP
 
   return (
     <div className="max-w-2xl mx-auto">
-      <nav className="meta mb-5 flex items-center gap-1.5 text-[13px]">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
         <ChevronRight size={13} />
         <span style={{ color: "var(--accent)" }}>通知中心</span>
diff --git a/frontend/app/p/[slug]/page.tsx b/frontend/app/p/[slug]/page.tsx
index 19c1306..b2c8494 100644
--- a/frontend/app/p/[slug]/page.tsx
+++ b/frontend/app/p/[slug]/page.tsx
@@ -97,6 +97,15 @@ export default async function SitePagePublic({ params }: PageProps) {
       <SiteDocShell
         left={left}
         right={right}
+        breadcrumb={
+          <SiteDocBreadcrumb
+            items={[
+              { href: "/", label: "首页" },
+              { label: "关于本站" },
+              { label: p.title },
+            ]}
+          />
+        }
         mobileExtras={
           <>
             {toc.length > 0 ? <PostToc items={toc} /> : null}
@@ -107,14 +116,6 @@ export default async function SitePagePublic({ params }: PageProps) {
           </>
         }
       >
-        <SiteDocBreadcrumb
-          items={[
-            { href: "/", label: "首页" },
-            { label: "关于本站" },
-            { label: p.title },
-          ]}
-        />
-
         <article className="panel p-6 sm:p-8">
           <SiteDocMasthead
             kicker="官方文件"
diff --git a/frontend/app/page.tsx b/frontend/app/page.tsx
index d23125f..fcf480b 100644
--- a/frontend/app/page.tsx
+++ b/frontend/app/page.tsx
@@ -17,6 +17,7 @@ import PostRow from "@/components/PostRow";
 import FeedTabs, { type FeedSort } from "@/components/home/FeedTabs";
 import FeedNewTip from "@/components/home/FeedNewTip";
 import { WorkspaceNav, WorkspaceNavMobile } from "@/components/home/WorkspaceNav";
+import { SiteDocBreadcrumb } from "@/components/site-doc";
 import { CommunityPanels, CommunityPanelsMobile } from "@/components/home/CommunityPanels";
 import {
   BoardPanels,
@@ -166,11 +167,21 @@ export default async function HomePage({ searchParams }: HomePageProps) {
     <div>
       <WorkspaceNavMobile boards={boards} user={user} />
 
-      <div className="flex items-start gap-6 xl:gap-7 mt-5 lg:mt-0">
+      <div className={`flex items-start gap-6 xl:gap-7 ${activeBoard ? "" : "mt-5 lg:mt-0"}`}>
         <WorkspaceNav boards={boards} user={user} />
 
         {/* 中栏：tab 栏与帖子流为同一个整体卡片（左栏切板块只替换这一列） */}
-        <div className="flex-1 min-w-0">
+        <div className="flex-1 min-w-0 relative">
+          {activeBoard ? (
+            <div className="mt-4 lg:mt-0">
+              <SiteDocBreadcrumb
+                items={[
+                  { href: "/", label: "首页" },
+                  { label: activeBoard.name },
+                ]}
+              />
+            </div>
+          ) : null}
           <h1 className="sr-only">{heading}</h1>
 
           <div className="panel overflow-hidden">
diff --git a/frontend/app/points/PointsClient.tsx b/frontend/app/points/PointsClient.tsx
index b418820..e4f3b02 100644
--- a/frontend/app/points/PointsClient.tsx
+++ b/frontend/app/points/PointsClient.tsx
@@ -42,7 +42,7 @@ export default function PointsClient() {
 
   return (
     <div className="max-w-3xl mx-auto">
-      <nav className="meta mb-5 flex items-center gap-1.5 text-[13px]">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">
           首页
         </Link>
diff --git a/frontend/app/post/[id]/edit/page.tsx b/frontend/app/post/[id]/edit/page.tsx
index 44441d5..fe6f282 100644
--- a/frontend/app/post/[id]/edit/page.tsx
+++ b/frontend/app/post/[id]/edit/page.tsx
@@ -39,7 +39,7 @@ export default async function EditPostPage({ params }: PageProps) {
       data-compose-viewport
       className="j13-compose-page w-full"
     >
-      <nav className="meta mb-4 flex items-center gap-1.5 flex-wrap text-[13px] shrink-0">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 flex-wrap text-[12.5px] leading-none shrink-0">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">
           首页
         </Link>
diff --git a/frontend/app/post/[id]/page.tsx b/frontend/app/post/[id]/page.tsx
index 88369cb..42b959b 100644
--- a/frontend/app/post/[id]/page.tsx
+++ b/frontend/app/post/[id]/page.tsx
@@ -30,6 +30,7 @@ import CommentSection from "@/components/CommentSection";
 import PostActions from "@/components/PostActions";
 import LikeButton from "@/components/LikeButton";
 import PostToc from "@/components/PostToc";
+import { SiteDocBreadcrumb } from "@/components/site-doc";
 import MobilePostBar from "@/components/MobilePostBar";
 import Avatar from "@/components/Avatar";
 import RoleBadge from "@/components/RoleBadge";
@@ -557,7 +558,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
   return (
     <>
       <script type="application/ld+json" dangerouslySetInnerHTML={{ __html: JSON.stringify(jsonLd) }} />
-      {/* 移动端浮动操作条：长文阅读中途可点赞 / 跳评论 / 回顶部 */}
+      {/* 移动端浮动操作条：长文读到中途可点赞 / 跳评论框 */}
       <MobilePostBar
         postId={id}
         liked={post.liked}
@@ -571,7 +572,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
         {/* sticky 必须直接挂在 grid 子项上：aside 作为 grid item，其可粘行程
             等于行高（正文高度）；内层包裹会被 items-start 缩成内容高而失去行程 */}
         <aside
-          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="帖子导航"
         >
           <BoardCard board={post.board} />
@@ -580,16 +581,14 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
         </aside>
 
         {/* 中栏：正文主体 */}
-        <div className="min-w-0">
+        <div className="min-w-0 relative">
+          <SiteDocBreadcrumb
+            items={[
+              { href: "/", label: "首页" },
+              { href: `/?board=${post.board.id}`, label: post.board.name },
+            ]}
+          />
           <article className="w-full">
-            <nav className="meta mb-5 flex items-center gap-1.5 flex-wrap text-[13px]">
-              <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
-              <ChevronRight size={13} />
-              <Link href={`/?board=${post.board.id}`} className="hover:text-[var(--accent)] transition-colors">
-                {post.board.name}
-              </Link>
-            </nav>
-
             <div className="panel p-6 sm:p-8">
               <header className="mb-7">
                 <div className="flex items-center gap-2 mb-4 flex-wrap">
@@ -810,7 +809,7 @@ export default async function PostDetailPage({ params, searchParams }: PageProps
 
         {/* 右栏：目录 / 同板块热议（阅读数据已并入标题下元信息条） */}
         <aside
-          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="阅读辅助"
         >
           {toc.length > 0 && <PostToc items={toc} />}
diff --git a/frontend/app/u/[id]/page.tsx b/frontend/app/u/[id]/page.tsx
index 877d868..e057dc0 100644
--- a/frontend/app/u/[id]/page.tsx
+++ b/frontend/app/u/[id]/page.tsx
@@ -121,7 +121,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
 
   return (
     <div>
-      <nav className="meta mb-5 flex items-center gap-1.5 text-[13px]">
+      <nav className="meta -mt-4 sm:-mt-6 mb-1.5 flex items-center gap-1 text-[12.5px] leading-none">
         <Link href="/" className="hover:text-[var(--accent)] transition-colors">首页</Link>
         <ChevronRight size={13} />
         <span style={{ color: "var(--accent)" }}>{u.nickname || u.username}</span>
@@ -276,7 +276,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
       {/* ═══ 三栏：左导航 / 中内容 / 右信息 ═══ */}
       <div className="grid grid-cols-1 items-start lg:grid-cols-[224px_minmax(0,1fr)_280px] xl:grid-cols-[240px_minmax(0,1fr)_300px] gap-x-6 xl:gap-x-7">
         {/* 左栏：Tab 导航 */}
-        <aside className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-24 flex-col gap-4">
+        <aside className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] flex-col gap-4">
           <section className="j13-module !p-2">
             <p
               className="px-2.5 pt-1.5 pb-2 text-[11.5px] font-bold tracking-[0.08em] uppercase"
@@ -341,7 +341,7 @@ export default async function UserPage({ params, searchParams }: UserPageProps)
         </div>
 
         {/* 右栏：账号信息 + 积分 + 安全提示 */}
-        <aside className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden">
+        <aside className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden">
           <section className="j13-module">
             <p className="j13-module-title mb-3">
               <Hash size={15} style={{ color: "var(--accent)" }} /> 账号信息
diff --git a/frontend/components/BackToTop.tsx b/frontend/components/BackToTop.tsx
new file mode 100644
index 0000000..60470cb
--- /dev/null
+++ b/frontend/components/BackToTop.tsx
@@ -0,0 +1,39 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { ArrowUp } from "lucide-react";
+import { scrollBehavior } from "@/lib/motion";
+
+const SHOW_AFTER = 400;
+
+/**
+ * 公开站长页回顶：滚过阈值后右下角浮现。
+ * 短页自然不出现；帖子移动端底栏时抬高，避免叠住操作条。
+ */
+export default function BackToTop({
+  liftForMobileBar = false,
+}: {
+  liftForMobileBar?: boolean;
+}) {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const onScroll = () => setVisible(window.scrollY > SHOW_AFTER);
+    onScroll();
+    window.addEventListener("scroll", onScroll, { passive: true });
+    return () => window.removeEventListener("scroll", onScroll);
+  }, []);
+
+  if (!visible) return null;
+
+  return (
+    <button
+      type="button"
+      className={`j13-back-top j13-toast-in${liftForMobileBar ? " is-lifted" : ""}`}
+      onClick={() => window.scrollTo({ top: 0, behavior: scrollBehavior() })}
+      aria-label="回到顶部"
+    >
+      <ArrowUp size={18} strokeWidth={2.25} aria-hidden />
+    </button>
+  );
+}
diff --git a/frontend/components/Header.tsx b/frontend/components/Header.tsx
index d493349..0aa79d0 100644
--- a/frontend/components/Header.tsx
+++ b/frontend/components/Header.tsx
@@ -204,7 +204,7 @@ export default function Header({
         WebkitBackdropFilter: "blur(14px)",
       }}
     >
-      <div className="max-w-[1440px] mx-auto px-4 sm:px-6 h-[68px] flex items-center gap-4 lg:gap-7">
+      <div className="max-w-[1440px] mx-auto px-4 sm:px-6 h-[var(--header-bar)] flex items-center gap-4 lg:gap-7">
         {/* 左组：品牌 + 主导航 */}
         <div className="flex items-center gap-3 lg:gap-5 shrink-0">
           <Link href="/" className="flex items-center gap-2.5 group">
diff --git a/frontend/components/MobilePostBar.tsx b/frontend/components/MobilePostBar.tsx
index 25b1ad8..fedb720 100644
--- a/frontend/components/MobilePostBar.tsx
+++ b/frontend/components/MobilePostBar.tsx
@@ -1,13 +1,12 @@
 "use client";
 
-import { useEffect, useState } from "react";
-import { ArrowUp, MessageSquare } from "lucide-react";
+import { MessageSquare } from "lucide-react";
 import LikeButton from "./LikeButton";
 import { scrollBehavior } from "@/lib/motion";
 
 /**
  * 帖子详情移动端浮动操作条（lg 以下显示）：
- * 长文读到中途也能点赞 / 跳评论框 / 回顶部，不必滚到文末。
+ * 长文读到中途也能点赞 / 跳评论框；回顶由全局 BackToTop 承担。
  */
 export default function MobilePostBar({
   postId,
@@ -20,15 +19,6 @@ export default function MobilePostBar({
   likeCount: number;
   allowComments?: boolean;
 }) {
-  const [showTop, setShowTop] = useState(false);
-
-  useEffect(() => {
-    const onScroll = () => setShowTop(window.scrollY > 400);
-    onScroll();
-    window.addEventListener("scroll", onScroll, { passive: true });
-    return () => window.removeEventListener("scroll", onScroll);
-  }, []);
-
   const scrollToComments = () => {
     document.getElementById("comment-input")?.scrollIntoView({
       behavior: scrollBehavior(),
@@ -36,13 +26,6 @@ export default function MobilePostBar({
     });
   };
 
-  const scrollToTop = () => {
-    window.scrollTo({
-      top: 0,
-      behavior: scrollBehavior(),
-    });
-  };
-
   return (
     <div className="lg:hidden fixed bottom-4 left-1/2 -translate-x-1/2 z-40" role="navigation" aria-label="阅读操作">
       <div
@@ -66,21 +49,9 @@ export default function MobilePostBar({
             <MessageSquare size={18} />
           </button>
         ) : null}
-        {/* 放大点赞按钮的触控区域（样式见 globals.css .j13-bar-like） */}
         <div className="j13-bar-like">
           <LikeButton postId={postId} liked={liked} likeCount={likeCount} variant="compact" />
         </div>
-        {showTop && (
-          <button
-            type="button"
-            onClick={scrollToTop}
-            className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--accent-soft)] focus-visible:bg-[var(--accent-soft)] j13-toast-in"
-            style={{ color: "var(--ink-2)" }}
-            aria-label="回到顶部"
-          >
-            <ArrowUp size={18} />
-          </button>
-        )}
       </div>
     </div>
   );
diff --git a/frontend/components/SiteChrome.tsx b/frontend/components/SiteChrome.tsx
index cd85cc1..608819f 100644
--- a/frontend/components/SiteChrome.tsx
+++ b/frontend/components/SiteChrome.tsx
@@ -4,6 +4,7 @@ import { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { usePathname } from "next/navigation";
 import Header from "@/components/Header";
 import Footer from "@/components/Footer";
+import BackToTop from "@/components/BackToTop";
 import type { User } from "@/lib/api";
 
 function pathIsAdmin(pathname: string | null) {
@@ -11,6 +12,17 @@ function pathIsAdmin(pathname: string | null) {
   return pathname === "/admin" || pathname.startsWith("/admin/");
 }
 
+function pathIsChat(pathname: string | null) {
+  if (!pathname) return false;
+  return pathname === "/chat" || pathname.startsWith("/chat/");
+}
+
+/** 帖子详情（非 edit）移动端有底栏，回顶需抬高 */
+function pathIsPostDetail(pathname: string | null) {
+  if (!pathname) return false;
+  return /^\/post\/[^/]+$/.test(pathname);
+}
+
 /**
  * 公开站 chrome 闸门：/admin 不渲染 Header/Footer/1440 main。
  * 首屏用 middleware 的 x-pathname 初值保证 hydration 一致；
@@ -63,6 +75,9 @@ export default function SiteChrome({
     return <>{children}</>;
   }
 
+  // 聊天页锁视口、主滚动在对话区，窗口回顶无意义
+  const showBackToTop = !pathIsChat(pathname);
+
   return (
     <>
       <Header
@@ -76,11 +91,14 @@ export default function SiteChrome({
       <main
         id="main"
         tabIndex={-1}
-        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-9 outline-none"
+        className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-[var(--main-pad-y)] outline-none"
       >
         {children}
       </main>
       <Footer siteName={siteName} />
+      {showBackToTop ? (
+        <BackToTop liftForMobileBar={pathIsPostDetail(pathname)} />
+      ) : null}
     </>
   );
 }
diff --git a/frontend/components/home/BoardPanels.tsx b/frontend/components/home/BoardPanels.tsx
index 6e4faf2..a22de81 100644
--- a/frontend/components/home/BoardPanels.tsx
+++ b/frontend/components/home/BoardPanels.tsx
@@ -283,8 +283,11 @@ function BoardPanelsBody(props: BoardPanelsProps) {
 
 export function BoardPanels(props: BoardPanelsProps) {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="板块面板">
-      <div className="sticky top-24 space-y-4">
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="板块面板"
+    >
+      <div className="space-y-4">
         <BoardPanelsBody {...props} />
       </div>
     </aside>
@@ -302,18 +305,19 @@ export function BoardPanelsMobile(props: BoardPanelsProps) {
 /** 板块不存在时的简卡 */
 export function BoardMissingPanel() {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="板块面板">
-      <div className="sticky top-24">
-        <section className="j13-module">
-          <p className="j13-module-title mb-2">
-            <LayoutGrid size={15} style={{ color: "var(--ink-3)" }} /> 板块
-          </p>
-          <p className="meta">板块不存在或已删除</p>
-          <Link href="/" className="btn btn-line btn-sm mt-3 w-full justify-center">
-            返回全部讨论
-          </Link>
-        </section>
-      </div>
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="板块面板"
+    >
+      <section className="j13-module">
+        <p className="j13-module-title mb-2">
+          <LayoutGrid size={15} style={{ color: "var(--ink-3)" }} /> 板块
+        </p>
+        <p className="meta">板块不存在或已删除</p>
+        <Link href="/" className="btn btn-line btn-sm mt-3 w-full justify-center">
+          返回全部讨论
+        </Link>
+      </section>
     </aside>
   );
 }
diff --git a/frontend/components/home/CommunityPanels.tsx b/frontend/components/home/CommunityPanels.tsx
index d01a306..aab004a 100644
--- a/frontend/components/home/CommunityPanels.tsx
+++ b/frontend/components/home/CommunityPanels.tsx
@@ -450,8 +450,11 @@ function CommunityPanelsBody({
 
 export function CommunityPanels(props: PanelsProps) {
   return (
-    <aside className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]" aria-label="社区面板">
-      <div className="sticky top-24 space-y-4">
+    <aside
+      className="hidden w-[280px] shrink-0 lg:block xl:w-[300px]"
+      aria-label="社区面板"
+    >
+      <div className="space-y-4">
         <CommunityPanelsBody {...props} />
       </div>
     </aside>
diff --git a/frontend/components/home/WorkspaceNav.tsx b/frontend/components/home/WorkspaceNav.tsx
index 2bf4d34..4cbb670 100644
--- a/frontend/components/home/WorkspaceNav.tsx
+++ b/frontend/components/home/WorkspaceNav.tsx
@@ -16,8 +16,12 @@ export function WorkspaceNav({ boards }: { boards: BoardWithCount[]; user?: User
   const activeBoard = Number(params.get("board")) || 0;
 
   return (
-    <nav className="hidden lg:block w-56 xl:w-60 shrink-0" aria-label="工作台导航">
-      <div className="sticky top-24 space-y-5">
+    // sticky 必须挂在 flex 子项上：父级 items-start 会把内层缩成内容高，失去粘滞行程
+    <nav
+      className="hidden lg:block w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] max-h-[var(--rail-max-h)] j13-rail-scroll"
+      aria-label="工作台导航"
+    >
+      <div className="space-y-5">
         {/* 「全部讨论」是重置筛选的动作入口，与板块目录分区展示；卡片样式与下方板块一致 */}
         <div className="space-y-0.5">
           <Link href="/" className="j13-bcard" aria-current={activeBoard === 0 ? "page" : undefined}>
diff --git a/frontend/components/site-doc/SiteDocBreadcrumb.tsx b/frontend/components/site-doc/SiteDocBreadcrumb.tsx
index eb793e9..c3b2e10 100644
--- a/frontend/components/site-doc/SiteDocBreadcrumb.tsx
+++ b/frontend/components/site-doc/SiteDocBreadcrumb.tsx
@@ -3,24 +3,29 @@ import { ChevronRight } from "lucide-react";
 
 export type Crumb = { href?: string; label: string };
 
-/** 公文页面包屑 */
+/** 位置指引：桌面叠在中栏卡片上方的内边距里，不拉低卡片、不对齐到左栏 */
 export default function SiteDocBreadcrumb({ items }: { items: Crumb[] }) {
   return (
-    <nav className="meta mb-5 flex items-center gap-1.5 flex-wrap text-[13px]" aria-label="面包屑">
+    <nav
+      className="meta mb-2 flex items-center gap-1 flex-wrap text-[12.5px] leading-none lg:absolute lg:bottom-full lg:left-0 lg:right-0 lg:mb-0 lg:pb-2 lg:pl-1"
+      aria-label="面包屑"
+    >
       {items.map((item, i) => {
         const last = i === items.length - 1;
+        const cls = last ? "truncate" : "shrink-0";
         return (
-          <span key={`${item.label}-${i}`} className="inline-flex items-center gap-1.5 min-w-0">
-            {i > 0 ? <ChevronRight size={13} className="shrink-0" /> : null}
-            {item.href && !last ? (
-              <Link href={item.href} className="hover:text-[var(--accent)] transition-colors shrink-0">
+          <span key={`${item.label}-${i}`} className="inline-flex items-center gap-1 min-w-0">
+            {i > 0 ? <ChevronRight size={12} className="shrink-0 opacity-70" aria-hidden /> : null}
+            {item.href ? (
+              <Link
+                href={item.href}
+                className={`hover:text-[var(--accent)] transition-colors ${cls}`}
+                style={last ? { color: "var(--accent)" } : undefined}
+              >
                 {item.label}
               </Link>
             ) : (
-              <span
-                className={last ? "truncate" : "shrink-0"}
-                style={last ? { color: "var(--accent)" } : undefined}
-              >
+              <span className={cls} style={last ? { color: "var(--accent)" } : undefined}>
                 {item.label}
               </span>
             )}
diff --git a/frontend/components/site-doc/SiteDocShell.tsx b/frontend/components/site-doc/SiteDocShell.tsx
index 55cebe8..fe59f1b 100644
--- a/frontend/components/site-doc/SiteDocShell.tsx
+++ b/frontend/components/site-doc/SiteDocShell.tsx
@@ -8,11 +8,14 @@ export default function SiteDocShell({
   left,
   right,
   children,
+  breadcrumb,
   mobileExtras,
 }: {
   left?: ReactNode;
   right?: ReactNode;
   children: ReactNode;
+  /** 中栏卡片上方的位置指引；桌面绝对定位，不占栅格高度 */
+  breadcrumb?: ReactNode;
   /** 窄屏下沉到正文之后的侧栏副本 */
   mobileExtras?: ReactNode;
 }) {
@@ -20,16 +23,19 @@ export default function SiteDocShell({
     <>
       <div className="grid grid-cols-1 items-start lg:grid-cols-[224px_minmax(0,1fr)_280px] xl:grid-cols-[240px_minmax(0,1fr)_300px] gap-x-6 xl:gap-x-7">
         <aside
-          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-56 xl:w-60 shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="站点导航"
         >
           {left}
         </aside>
 
-        <div className="min-w-0">{children}</div>
+        <div className="min-w-0 relative">
+          {breadcrumb}
+          {children}
+        </div>
 
         <aside
-          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-24 flex-col gap-4 max-h-[calc(100vh_-_7rem)] overflow-hidden"
+          className="hidden lg:flex w-[280px] xl:w-[300px] shrink-0 sticky top-[var(--rail-top)] flex-col gap-4 max-h-[var(--rail-max-h)] overflow-hidden"
           aria-label="阅读辅助"
         >
           {right}