diff --git a/client/electron/src/components/MailView.tsx b/client/electron/src/components/MailView.tsx
index 0105c39..dac4bee 100644
--- a/client/electron/src/components/MailView.tsx
+++ b/client/electron/src/components/MailView.tsx
@@ -14,7 +14,8 @@ import {
 } from '../lib/replyTarget';
 import * as api from '../api/client';
 import type { Mail } from '../types';
-import { MailIcon, ShieldIcon, PersonIcon, BotIcon, CheckIcon, CloseIcon, ForwardIcon, TreeIcon, TagIcon, GaugeIcon } from './icons';
+import { MailIcon, ShieldIcon, PersonIcon, BotIcon, CheckIcon, CloseIcon, ForwardIcon, TreeIcon, TagIcon, GaugeIcon, ChevronRightIcon, ChatBubbleIcon } from './icons';
+import { useIsNarrow } from '../hooks/useIsNarrow';
 import AddressInput from './AddressInput';
 import { AttachmentList, AttachmentPicker, type PendingAttachment } from './Attachments';
 import ThreadView from './ThreadView';
@@ -34,6 +35,7 @@ export default function MailView() {
   const [forwarding, setForwarding] = useState<Mail | null>(null);
   // 正在看哪封邮件的对话树；null = 看正常的邮件视图
   const [threadOf, setThreadOf] = useState<string | null>(null);
+  const narrow = useIsNarrow();
 
   // 切换邮件时关掉树视图：树是针对某封邮件的，留着会显示上一封的线索
   const currentMailID = currentMail?.mail_id;
@@ -49,24 +51,33 @@ export default function MailView() {
     const sessionTarget = sessionReplyTarget(currentSessionMails, currentSession, me);
     return (
       <div className="flex-1 min-w-0 flex flex-col bg-gray-50">
-        <div className="px-4 md:px-6 py-3 border-b border-gray-200 bg-white">
+        <CollapsibleHeader
+          narrow={narrow}
+          lead={<BackButton label="会话" />}
+          title={
+            currentSession.subject ||
+            (currentSession.session_alias ? `.${currentSession.session_alias}` : '(未命名会话)')
+          }
+          meta={
+            <>
+              <StatusBadge status={currentSession.status} />
+              <span className="text-xs text-gray-400 shrink-0">
+                {currentSessionMails.length} 封
+              </span>
+            </>
+          }
+        >
           <div className="flex items-center gap-2 flex-wrap">
-            <BackButton label="会话" />
-            <span className="text-sm font-semibold text-gray-900 font-mono">
+            <span className="text-xs font-semibold text-gray-900 font-mono">
               {currentSession.session_alias
                 ? `.${currentSession.session_alias}`
                 : '(未命名会话)'}
             </span>
-            <StatusBadge status={currentSession.status} />
-            <span className="text-xs text-gray-400">
-              {currentSessionMails.length} 封
-            </span>
             <div className="flex-1" />
             <PermissionEditor />
             <BudgetEditor />
           </div>
-          <p className="text-xs text-gray-500 mt-0.5">{currentSession.subject}</p>
-        </div>
+        </CollapsibleHeader>
         <RenameProposalBar />
         <div className="flex-1 overflow-y-auto px-4 md:px-6 py-4 space-y-3">
           {currentSessionMails.map(m => (
@@ -463,22 +474,27 @@ function Header({
   );
 
   return (
-    <div className="px-4 md:px-6 py-3 md:py-4 border-b border-gray-200">
+    <CollapsibleHeader
+      narrow={narrow}
+      lead={<BackButton />}
+      title={mail.subject}
+      meta={
+        <>
+          {mail.status === 'unread' && (
+            <span className="shrink-0 px-1.5 py-0.5 rounded bg-blue-100 text-blue-700 text-3xs font-medium">
+              未读
+            </span>
+          )}
+          {mail.mail_type === 'permission_request' && (
+            <span className="shrink-0 inline-flex items-center gap-1 px-1.5 py-0.5 rounded bg-orange-100 text-orange-700 text-3xs font-medium">
+              <ShieldIcon className="w-3 h-3" />
+              权限请求
+            </span>
+          )}
+        </>
+      }
+    >
       <div className="flex items-center gap-2 mb-1.5 flex-wrap">
-        <BackButton />
-        <h2 className="text-sm font-semibold text-gray-900 min-w-0 break-words">{mail.subject}</h2>
-        {mail.status === 'unread' && (
-          <span className="px-1.5 py-0.5 rounded bg-blue-100 text-blue-700 text-3xs font-medium">
-            未读
-          </span>
-        )}
-        {mail.mail_type === 'permission_request' && (
-          <span className="inline-flex items-center gap-1 px-1.5 py-0.5 rounded bg-orange-100 text-orange-700 text-3xs font-medium">
-            <ShieldIcon className="w-3 h-3" />
-            权限请求
-          </span>
-        )}
-        <div className="flex-1" />
         {mail.status === 'unread' && (
           <button onClick={onRead} className="tap text-xs text-blue-600 hover:underline">
             标记已读
@@ -511,6 +527,69 @@ function Header({
         )}
         <Row label="时间">{time}</Row>
       </dl>
+    </CollapsibleHeader>
+  );
+}
+
+/**
+ * 窄屏可折叠头部（邮件详情 / 会话）。
+ *
+ * 用户（2026-09-14）：「窄屏页面阅读邮件时，顶部的邮件信息和底部的输入框等
+ * 占用了绝大部分页面，用户只能通过中间的一小块看邮件，体验十分不好，
+ * 这些位置应当有一个自动隐藏的逻辑，比如上面缩为窄栏，只显示邮件标题，
+ * 点击展开显示完整信息」。
+ *
+ * 做法：窄屏**默认收起**，只留一行标题（+ 未读/权限这类必须常驻的状态点），
+ * 点标题行展开完整信息与操作。宽屏没有这个矛盾（横向空间够），所以宽屏恒展开、
+ * 不引入多余点击 —— 同一套头部代码，两种默认值。
+ *
+ * 跨过断点时要重置（宽 → 窄应自动收起），否则「自动隐藏」只在首次生效。
+ */
+function CollapsibleHeader({
+  narrow,
+  lead,
+  title,
+  meta,
+  children
+}: {
+  narrow: boolean;
+  /** 标题行最左侧的固定元素（返回键）；它不能套在 toggle 按钮里（button 嵌 button） */
+  lead?: React.ReactNode;
+  title: React.ReactNode;
+  /** 收起态也要常驻的状态点（未读 / 权限请求 / 会话状态） */
+  meta?: React.ReactNode;
+  /** 展开后才显示的内容（收发件人、时间、操作按钮…） */
+  children: React.ReactNode;
+}) {
+  const [open, setOpen] = useState(!narrow);
+  useEffect(() => setOpen(!narrow), [narrow]);
+
+  return (
+    <div className="shrink-0 px-4 md:px-6 py-2 md:py-3.5 border-b border-gray-200 bg-white">
+      <div className="flex items-center gap-2">
+        {lead}
+        <button
+          type="button"
+          // 宽屏没有可展开的东西（已经全展开了），此时点它不该有任何副作用
+          onClick={() => narrow && setOpen(o => !o)}
+          aria-expanded={open}
+          aria-label={narrow ? (open ? '收起邮件信息' : '展开邮件信息') : undefined}
+          className={`min-w-0 flex-1 flex items-center gap-2 text-left ${narrow ? 'tap' : 'cursor-default'}`}
+        >
+          <span className="min-w-0 flex-1 truncate text-sm font-semibold text-gray-900">
+            {title}
+          </span>
+          {meta}
+          {narrow && (
+            <ChevronRightIcon
+              className={`w-4 h-4 shrink-0 text-gray-400 transition-transform duration-200 motion-reduce:transition-none ${
+                open ? 'rotate-90' : ''
+              }`}
+            />
+          )}
+        </button>
+      </div>
+      {open && <div className="mt-1.5">{children}</div>}
     </div>
   );
 }
@@ -802,9 +881,33 @@ function ReplyBar({
   const budget = useSessionStore(s => s.budget);
   const setBudget = useSessionStore(s => s.setBudget);
   const me = useAuthStore(s => s.user?.username || '');
+  const narrow = useIsNarrow();
+  // 窄屏默认收起（只留悬浮球）；宽屏恒展开。跨断点时重置，否则“自动隐藏”只在首次生效。
+  const [open, setOpen] = useState(!narrow);
+  useEffect(() => setOpen(!narrow), [narrow]);
 
   if (!replyTo) return null;
 
+  // 窄屏收起态：一个悬浮球，点开才出现输入区。
+  //
+  // 窄屏上「顶部邮件信息 + 底部输入框」同时常驻会把可读区压成一条缝（用户实测反馈）。
+  // 输入框不是阅读时每刻都要用的东西，就该按需展开；球放右下、悬浮于底部导航之上，
+  // 与列表页的 ComposeFab 同一套观感，不新增一种视觉语言。
+  if (narrow && !open) {
+    return (
+      <button
+        type="button"
+        data-testid="reply-fab"
+        onClick={() => setOpen(true)}
+        title={`回复 ${target}`}
+        aria-label={`回复 ${target}`}
+        className="absolute bottom-4 right-4 z-20 w-14 h-14 rounded-full bg-blue-600 text-white shadow-lg flex items-center justify-center hover:bg-blue-700 active:bg-blue-800 transition-colors"
+      >
+        <ChatBubbleIcon className="w-6 h-6" />
+      </button>
+    );
+  }
+
   // 判据是「当前登录用户名」而不是字面量 'human'：后者是单用户时代的遗留，
   // 多用户下登录名可能是 jianf，判据恒为假 → 对端取成自己 → 信发给自己。
   const peer = mailCounterpart(replyTo, me);
@@ -868,6 +971,16 @@ function ReplyBar({
         >
           {ccOpen ? '收起抄送' : '抄送'}
         </button>
+        {narrow && (
+          <button
+            onClick={() => setOpen(false)}
+            className="tap inline-flex items-center gap-0.5 text-3xs text-gray-500 hover:text-blue-600"
+            aria-label="收起回复框"
+          >
+            收起
+            <ChevronRightIcon className="w-3 h-3 rotate-90" />
+          </button>
+        )}
       </div>
       {ccOpen && (
         <div className="mb-2">
@@ -883,6 +996,8 @@ function ReplyBar({
         value={body}
         onChange={e => setBody(e.target.value)}
         placeholder="回复内容（Markdown）"
+        // 窄屏是「点球展开」进来的，焦点直接落到输入框，少一次点击
+        autoFocus={narrow}
         className="w-full h-20 text-sm font-mono border border-gray-300 rounded-md p-3 resize-none focus:outline-none focus:ring-2 focus:ring-blue-100 focus:border-blue-400"
       />
       <div className="mt-2">
diff --git a/client/electron/src/components/icons.tsx b/client/electron/src/components/icons.tsx
index 9a47f2f..332fee4 100644
--- a/client/electron/src/components/icons.tsx
+++ b/client/electron/src/components/icons.tsx
@@ -417,3 +417,18 @@ export function MonitorIcon({ className = 'w-4 h-4' }: P) {
     </Svg>
   );
 }
+
+/**
+ * 聊天气泡：窄屏「展开回复输入框」的悬浮球图标。
+ *
+ * 为什么复用不了一个现成的：MailIcon 是信封（已读/收件语义）、ComposeIcon 是
+ * 铅笔加号（新建），而这里要表达的是「跟他聊一句」——就是回复这件事本身，
+ * 用一个气泡最不容易被误读成别的操作。
+ */
+export function ChatBubbleIcon({ className = 'w-4 h-4' }: P) {
+  return (
+    <Svg className={className}>
+      <path d="M21 11.5a8.4 8.4 0 0 1-9 8.4 9.4 9.4 0 0 1-3.3-.6L3 21l1.7-5a8.4 8.4 0 0 1-.7-3.4 8.4 8.4 0 0 1 8.4-8.4h.6a8.4 8.4 0 0 1 8 8z" />
+    </Svg>
+  );
+}
diff --git a/client/electron/test/manual/rotate-verify.mjs b/client/electron/test/manual/rotate-verify.mjs
index e048f87..51a909d 100644
--- a/client/electron/test/manual/rotate-verify.mjs
+++ b/client/electron/test/manual/rotate-verify.mjs
@@ -19,7 +19,11 @@ const record = (n, ok, note) => {
 const snap = () =>
   page.evaluate(() => {
     const de = document.documentElement;
-    const content = document.querySelector('.narrow-shell > *:not(.narrow-nav)');
+    // 排除底部导航**与侧边栏**：紧凑横屏下第一个子元素是 60px 侧栏（不是内容区），
+    // 不排除就会把侧栏当成内容区来判断排布（判据自己的错，实测踩到）
+    const content = document.querySelector(
+      '.narrow-shell > *:not(.narrow-nav):not(.bg-chrome-900)'
+    );
     const nav = document.querySelector('.narrow-nav')?.getBoundingClientRect();
     return {
       vw: innerWidth,
@@ -28,7 +32,15 @@ const snap = () =>
       overflowX: de.scrollWidth - de.clientWidth,
       contentCls: content ? content.className.toString().slice(0, 40) : '',
       navVisible: nav ? nav.bottom <= innerHeight + 1 && nav.top > 0 : false,
-      navInset: nav ? Math.round(nav.left) : null
+      navInset: nav ? Math.round(nav.left) : null,
+      hasBottomNav: !!document.querySelector('.narrow-nav'),
+      sideRail: (() => {
+        const r = document.querySelector('.narrow-shell > .bg-chrome-900');
+        if (!r) return null;
+        const b = r.getBoundingClientRect();
+        return { w: Math.round(b.width), h: Math.round(b.height), x: Math.round(b.x) };
+      })(),
+      listPaneW: Math.round((document.querySelector('.comm-pane')?.getBoundingClientRect().width) ?? 0)
     };
   });
 
@@ -37,6 +49,7 @@ try {
   await page.waitForTimeout(1500);
   const p1 = await snap();
   record('竖屏：覆盖式排布（NarrowStack）', /relative overflow-hidden/.test(p1.contentCls), p1.contentCls);
+  record('竖屏：导航在**底部**（不是侧边）', p1.hasBottomNav === true && p1.sideRail === null, `底部=${p1.hasBottomNav} 侧栏=${!!p1.sideRail}`);
   record('竖屏：--app-height 跟随视口', p1.appHeight === `${p1.vh}px`, `${p1.appHeight} vs ${p1.vh}`);
 
   await page.setViewportSize({ width: 844, height: 390 });
@@ -45,7 +58,17 @@ try {
   record('横屏：真的换了排布（并排，而不是原样）', /flex-1 min-h-0 flex$/.test(l.contentCls), l.contentCls);
   record('横屏：--app-height 重算（旋屏后不是旧值）', l.appHeight === '390px', l.appHeight);
   record('横屏：无横向溢出', l.overflowX <= 0, `溢出 ${l.overflowX}px`);
-  record('横屏：底部导航仍完整可见且浮着', l.navVisible && l.navInset >= 6, `left=${l.navInset}`);
+  // 用户后续追问：「导航栏在窄屏横屏情况下不也应当是在侧边吗？」⇒ 横屏用侧边栏
+  record(
+    '横屏：导航在**侧边**（60px 图标栏），底部导航不存在',
+    !!l.sideRail && l.sideRail.w === 60 && l.sideRail.x <= 12 && l.hasBottomNav === false,
+    `侧栏=${l.sideRail?.w}px x=${l.sideRail?.x} 底部导航=${l.hasBottomNav}`
+  );
+  record(
+    '横屏：列表栏固定 320（lg:断点在 844 宽下不生效，否则会吃掉整宽）',
+    l.listPaneW === 320,
+    `列表宽=${l.listPaneW}`
+  );
 
   await page.setViewportSize({ width: 390, height: 844 });
   await page.waitForTimeout(1300);