diff --git a/frontend/dashboard.css b/frontend/dashboard.css
index 41eacab..ab5031a 100644
--- a/frontend/dashboard.css
+++ b/frontend/dashboard.css
@@ -469,26 +469,28 @@ textarea { resize: vertical; min-height: 120px; }
 .issue-sheet-panel { width:min(560px,100%); height:100%; overflow:auto; padding:18px; background:#0b1526; border-left:1px solid #2a496e; }
 .issue-sheet-header { display:flex; align-items:center; justify-content:space-between; gap:10px; }
 .issue-sheet-header button { min-height:44px; }
-.mobile-issue-detail-nav, .mobile-pull-detail-nav { display:none; }
+.mobile-issue-detail-nav, .mobile-pull-detail-nav, .mobile-update-detail-nav { display:none; }
 @media (max-width:600px) {
-  .issue-sheet-panel, .pull-sheet-panel { padding-top:max(12px,env(safe-area-inset-top)); }
-  .mobile-issue-detail-nav, .mobile-pull-detail-nav {
+  .issue-sheet-panel, .pull-sheet-panel, .update-sheet-panel { padding-top:max(12px,env(safe-area-inset-top)); }
+  .mobile-issue-detail-nav, .mobile-pull-detail-nav, .mobile-update-detail-nav {
     position:sticky; top:env(safe-area-inset-top); z-index:6;
     display:grid; grid-template-columns:repeat(4,minmax(0,1fr)); gap:4px;
     margin:8px -6px 12px; padding:6px;
     background:rgba(11,21,38,.98); border-block:1px solid #2a496e;
   }
-  .mobile-issue-detail-nav button, .mobile-pull-detail-nav button {
+  .mobile-issue-detail-nav button, .mobile-pull-detail-nav button, .mobile-update-detail-nav button {
     min-width:0; min-height:44px; padding:4px; overflow-wrap:anywhere;
     border-color:transparent; font-size:12px;
   }
-  .mobile-issue-detail-nav button[aria-current="location"], .mobile-pull-detail-nav button[aria-current="location"] {
+  .mobile-issue-detail-nav button[aria-current="location"], .mobile-pull-detail-nav button[aria-current="location"], .mobile-update-detail-nav button[aria-current="location"] {
     border-color:#60a5fa; background:#17365a; color:#fff;
   }
   #issue-overview, #issue-conversation, #issue-comment, #issue-planning { scroll-margin-top:72px; }
   #pull-overview, #pull-conversation, #pull-comment, #pull-review { scroll-margin-top:72px; }
+  #update-conversation, #update-subject-context, #update-reply-workspace { scroll-margin-top:72px; }
 }
 @media (min-width:601px) { .mobile-issue-detail-nav, .mobile-pull-detail-nav { display:none; } }
+@media (min-width:601px) { .mobile-update-detail-nav { display:none; } }
 .completed-filed-actions { position:fixed; right:0; bottom:0; z-index:57; box-sizing:border-box; width:min(560px,100%); display:grid; grid-template-columns:minmax(0,1fr); align-items:center; gap:8px; margin:0; padding:10px 12px calc(10px + env(safe-area-inset-bottom)); border:1px solid #4ade80; border-radius:12px 0 0; background:rgba(11,21,38,.98); overflow-wrap:anywhere; }
 .completed-filed-actions[hidden] { display:none; }
 .completed-filed-actions button { min-height:44px; min-width:0; }
diff --git a/frontend/dashboard.js b/frontend/dashboard.js
index fc372c4..b50053b 100644
--- a/frontend/dashboard.js
+++ b/frontend/dashboard.js
@@ -1353,6 +1353,19 @@
     panel: qs('#update-sheet .update-sheet-panel'),
     jump: qs('#jump-update-new-activity'),
   });
+  const mobileUpdateDetailNavigation = createMobileUpdateDetailNavigation({
+    root:qs('#update-sheet .update-sheet-panel'),
+    buttons:Object.fromEntries(Array.from(document.querySelectorAll('[data-update-section]')).map(button => [button.dataset.updateSection, button])),
+    targets:{
+      conversation:qs('#update-conversation'),
+      context:qs('#update-subject-context'),
+      reply:qs('#update-reply-workspace'),
+    },
+    replyComposer:qs('#update-reply'),
+    jumpToNewActivity:() => updateReadPosition.jump(),
+    prefersReducedMotion:() => window.matchMedia('(prefers-reduced-motion: reduce)').matches,
+  });
+  mobileUpdateDetailNavigation.start();
   const issueDetailPosition = createWorkDetailPosition({ panel: qs('#issue-sheet .issue-sheet-panel') });
   const pullDetailPosition = createWorkDetailPosition({ panel: qs('#pull-sheet .pull-sheet-panel') });
   const reviewDetailPosition = createWorkDetailPosition({ panel: qs('#review-sheet .review-sheet-panel') });
@@ -1391,6 +1404,7 @@
       void updateVoiceReply.open('update:' + item.notification_id);
       selectedUpdateDetail = null;
       updateReadPosition.open(String(item.notification_id));
+      mobileUpdateDetailNavigation.reset();
       updateMentions.dismiss();
       qs('#update-sheet').classList.add('open');
       qs('.update-more-actions').open = false;
diff --git a/frontend/index.html b/frontend/index.html
index 3d3a878..9816f55 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -1098,6 +1098,12 @@
       </div>
     </div>
     <div id="update-triage-progress" class="update-triage-progress small" aria-live="polite" hidden></div>
+    <nav class="mobile-update-detail-nav" aria-label="Update sections">
+      <button type="button" data-update-section="activity">New activity</button>
+      <button type="button" data-update-section="conversation">Conversation</button>
+      <button type="button" data-update-section="context">Subject context</button>
+      <button type="button" data-update-section="reply">Reply</button>
+    </nav>
     <div id="update-gesture-status" class="update-gesture-status small" aria-live="polite">Swipe right to keep unread or left to mark read. Buttons remain available.</div>
     <div id="update-sheet-status" class="small" aria-live="polite">Choose an update.</div>
     <button class="update-retry" id="retry-update-load" type="button" hidden>Retry loading update</button>
@@ -1105,18 +1111,20 @@
       <span class="pill" id="update-subject-type">Update</span>
       <span class="pill" id="update-subject-state"></span>
     </div>
-    <h2>Full conversation</h2>
-    <div id="update-comments"></div>
-    <button class="update-new-activity-jump" id="jump-update-new-activity" type="button" hidden>Jump to new activity</button>
-    <button class="conversation-more" id="load-older-update-comments" type="button" hidden>Load older messages</button>
-    <div id="update-conversation-status" class="small" aria-live="assertive"></div>
-    <button class="update-retry" id="retry-update-conversation" type="button" hidden>Retry conversation</button>
-    <button class="conversation-actions-retry" id="retry-update-comment-actions" type="button" hidden>Retry comment tools</button>
-    <details>
+    <section id="update-conversation" tabindex="-1">
+      <h2>Full conversation</h2>
+      <div id="update-comments"></div>
+      <button class="update-new-activity-jump" id="jump-update-new-activity" type="button" hidden>Jump to new activity</button>
+      <button class="conversation-more" id="load-older-update-comments" type="button" hidden>Load older messages</button>
+      <div id="update-conversation-status" class="small" aria-live="assertive"></div>
+      <button class="update-retry" id="retry-update-conversation" type="button" hidden>Retry conversation</button>
+      <button class="conversation-actions-retry" id="retry-update-comment-actions" type="button" hidden>Retry comment tools</button>
+    </section>
+    <details id="update-subject-context">
       <summary><h2>Subject context</h2></summary>
       <div class="update-sheet-content muted markdown-content" id="update-subject-body"></div>
     </details>
-    <section class="update-reply" aria-labelledby="update-reply-title">
+    <section class="update-reply" id="update-reply-workspace" aria-labelledby="update-reply-title">
       <h2 id="update-reply-title">Reply</h2>
       <label for="update-reply" class="small">Continue this issue or pull-request conversation</label>
       <textarea id="update-reply" maxlength="10000" placeholder="Write a reply"></textarea>
@@ -1549,6 +1557,7 @@
 <script src="static/push-notifications.js"></script>
 <script src="static/issue-filing-receipt.js"></script>
 <script src="static/mobile-issue-detail-nav.js"></script>
+<script src="static/mobile-update-detail-nav.js"></script>
 <script src="static/dashboard.js"></script>
 </body>
 </html>
diff --git a/frontend/mobile-update-detail-nav.js b/frontend/mobile-update-detail-nav.js
new file mode 100644
index 0000000..70af14d
--- /dev/null
+++ b/frontend/mobile-update-detail-nav.js
@@ -0,0 +1,83 @@
+function createMobileUpdateDetailNavigation(options) {
+  const buttons = options.buttons || {};
+  const targets = options.targets || {};
+  const listeners = new Map();
+  const prefersReducedMotion = options.prefersReducedMotion || (() => false);
+  const targetNames = new Map(Object.entries(targets).map(([name, target]) => [target, name]));
+  let observer = null;
+
+  function select(name) {
+    Object.entries(buttons).forEach(([key, button]) => {
+      if (!button) return;
+      if (key === name) button.setAttribute('aria-current', 'location');
+      else button.removeAttribute('aria-current');
+    });
+  }
+
+  function navigate(name) {
+    if (name === 'activity') {
+      if (options.jumpToNewActivity) options.jumpToNewActivity();
+      select(name);
+      return true;
+    }
+    const target = targets[name];
+    if (!target) return false;
+    if (name === 'context') target.open = true;
+    target.scrollIntoView({
+      block:'start',
+      behavior:prefersReducedMotion() ? 'auto' : 'smooth',
+    });
+    if (name === 'reply' && options.replyComposer) {
+      options.replyComposer.focus({ preventScroll:true });
+    }
+    select(name);
+    return true;
+  }
+
+  function reset() {
+    if (targets.context) targets.context.open = false;
+    select('activity');
+  }
+
+  return {
+    start() {
+      Object.entries(buttons).forEach(([name, button]) => {
+        if (!button || listeners.has(button)) return;
+        const listener = event => {
+          event.preventDefault();
+          navigate(name);
+        };
+        listeners.set(button, listener);
+        button.addEventListener('click', listener);
+      });
+      reset();
+      const observe = options.observe || ((handler, observedTargets) => {
+        if (typeof IntersectionObserver === 'undefined') return null;
+        const instance = new IntersectionObserver(handler, {
+          root:options.root || null,
+          rootMargin:'-20% 0px -60% 0px',
+          threshold:[0, 0.25, 0.5, 0.75, 1],
+        });
+        observedTargets.forEach(target => instance.observe(target));
+        return instance;
+      });
+      observer = observe(entries => {
+        const visible = entries
+          .filter(entry => entry.isIntersecting && targetNames.has(entry.target))
+          .sort((left, right) => right.intersectionRatio - left.intersectionRatio)[0];
+        if (visible) select(targetNames.get(visible.target));
+      }, Array.from(targetNames.keys()).filter(Boolean));
+    },
+    stop() {
+      listeners.forEach((listener, button) => button.removeEventListener('click', listener));
+      listeners.clear();
+      if (observer) observer.disconnect();
+      observer = null;
+    },
+    navigate,
+    reset,
+    select,
+  };
+}
+
+if (typeof module !== 'undefined') module.exports = createMobileUpdateDetailNavigation;
diff --git a/frontend/service-worker.js b/frontend/service-worker.js
index 9889541..4d39e7c 100644
--- a/frontend/service-worker.js
+++ b/frontend/service-worker.js
@@ -78,6 +78,7 @@ const SHELL = [
   BASE + 'static/issue-filing-review.js',
   BASE + 'static/issue-sheet.js',
   BASE + 'static/mobile-issue-detail-nav.js',
+  BASE + 'static/mobile-update-detail-nav.js',
   BASE + 'static/checklist-conflict.js',
   BASE + 'static/voice-transcript-store.js',
   BASE + 'static/voice-issue-capture.js',
diff --git a/tests/test_mobile_update_detail_navigation.py b/tests/test_mobile_update_detail_navigation.py
new file mode 100644
index 0000000..3bab92d
--- /dev/null
+++ b/tests/test_mobile_update_detail_navigation.py
@@ -0,0 +1,153 @@
+import json
+import subprocess
+from pathlib import Path
+
+
+FRONTEND = Path(__file__).resolve().parents[1] / "frontend"
+CONTROLLER = FRONTEND / "mobile-update-detail-nav.js"
+
+
+def test_update_navigation_reuses_new_activity_opens_context_and_focuses_reply():
+    script = f"""
+const createNavigation = require({json.dumps(str(CONTROLLER))});
+class FakeElement {{
+  constructor(name) {{
+    this.name=name; this.listeners={{}}; this.attributes={{}}; this.open=false;
+    this.focuses=0; this.scrolls=[];
+  }}
+  addEventListener(name, callback) {{ this.listeners[name]=callback; }}
+  removeEventListener(name) {{ delete this.listeners[name]; }}
+  setAttribute(name, value) {{ this.attributes[name]=value; }}
+  removeAttribute(name) {{ delete this.attributes[name]; }}
+  focus() {{ this.focuses += 1; }}
+  scrollIntoView(options) {{ this.scrolls.push({{...options, openWhenScrolled:this.open}}); }}
+}}
+const names=['activity','conversation','context','reply'];
+const buttons=Object.fromEntries(names.map(name => [name,new FakeElement(name)]));
+const targets=Object.fromEntries(['conversation','context','reply'].map(name => [name,new FakeElement(name)]));
+const replyComposer=new FakeElement('composer');
+let activityJumps=0;
+const navigation=createNavigation({{
+  buttons, targets, replyComposer,
+  jumpToNewActivity() {{ activityJumps += 1; }},
+  prefersReducedMotion:() => true,
+}});
+navigation.start();
+navigation.navigate('conversation');
+navigation.navigate('context');
+const contextOpenAfterNavigation=targets.context.open;
+navigation.navigate('reply');
+navigation.navigate('activity');
+const currentAfterActivity=Object.fromEntries(Object.entries(buttons).map(([name,button]) => [name,button.attributes['aria-current'] || null]));
+navigation.reset();
+const contextOpenAfterReset=targets.context.open;
+const currentAfterReset=Object.fromEntries(Object.entries(buttons).map(([name,button]) => [name,button.attributes['aria-current'] || null]));
+navigation.stop();
+process.stdout.write(JSON.stringify({{
+  activityJumps,
+  conversation:targets.conversation.scrolls,
+  context:targets.context.scrolls,
+  contextOpen:contextOpenAfterNavigation,
+  contextOpenAfterReset,
+  reply:targets.reply.scrolls,
+  replyFocuses:replyComposer.focuses,
+  currentAfterActivity,
+  currentAfterReset,
+  listenersAfterStop:Object.values(buttons).map(button => Object.keys(button.listeners).length),
+}}));
+"""
+    result = subprocess.run(["node", "-e", script], capture_output=True, text=True)
+
+    assert result.returncode == 0, result.stderr
+    assert json.loads(result.stdout) == {
+        "activityJumps": 1,
+        "conversation": [{"block": "start", "behavior": "auto", "openWhenScrolled": False}],
+        "context": [{"block": "start", "behavior": "auto", "openWhenScrolled": True}],
+        "contextOpen": True,
+        "contextOpenAfterReset": False,
+        "reply": [{"block": "start", "behavior": "auto", "openWhenScrolled": False}],
+        "replyFocuses": 1,
+        "currentAfterActivity": {
+            "activity": "location",
+            "conversation": None,
+            "context": None,
+            "reply": None,
+        },
+        "currentAfterReset": {
+            "activity": "location",
+            "conversation": None,
+            "context": None,
+            "reply": None,
+        },
+        "listenersAfterStop": [0, 0, 0, 0],
+    }
+
+
+def test_update_navigation_tracks_visible_sections_and_disconnects_observer():
+    script = f"""
+const createNavigation = require({json.dumps(str(CONTROLLER))});
+const makeElement=name => ({{
+  name, attributes:{{}},
+  addEventListener() {{}}, removeEventListener() {{}},
+  setAttribute(key,value) {{ this.attributes[key]=value; }},
+  removeAttribute(key) {{ delete this.attributes[key]; }},
+}});
+const buttons=Object.fromEntries(['activity','conversation','context','reply'].map(name => [name,makeElement(name)]));
+const targets=Object.fromEntries(['conversation','context','reply'].map(name => [name,makeElement(name)]));
+let callback;
+let disconnected=0;
+const navigation=createNavigation({{
+  buttons, targets,
+  observe(handler) {{ callback=handler; return {{disconnect() {{ disconnected += 1; }}}}; }},
+}});
+navigation.start();
+callback([
+  {{target:targets.conversation,isIntersecting:true,intersectionRatio:.2}},
+  {{target:targets.reply,isIntersecting:true,intersectionRatio:.8}},
+]);
+const current=Object.fromEntries(Object.entries(buttons).map(([name,button]) => [name,button.attributes['aria-current'] || null]));
+navigation.stop();
+process.stdout.write(JSON.stringify({{current,disconnected}}));
+"""
+    result = subprocess.run(["node", "-e", script], capture_output=True, text=True)
+
+    assert result.returncode == 0, result.stderr
+    assert json.loads(result.stdout) == {
+        "current": {
+            "activity": None,
+            "conversation": None,
+            "context": None,
+            "reply": "location",
+        },
+        "disconnected": 1,
+    }
+
+
+def test_update_sheet_wires_a_mobile_only_safe_area_navigation_rail():
+    html = (FRONTEND / "index.html").read_text()
+    css = (FRONTEND / "dashboard.css").read_text()
+    dashboard_js = (FRONTEND / "dashboard.js").read_text()
+    service_worker = (FRONTEND / "service-worker.js").read_text()
+
+    assert '<nav class="mobile-update-detail-nav" aria-label="Update sections">' in html
+    for name, label in (
+        ("activity", "New activity"),
+        ("conversation", "Conversation"),
+        ("context", "Subject context"),
+        ("reply", "Reply"),
+    ):
+        assert f'data-update-section="{name}"' in html
+        assert f">{label}</button>" in html
+    assert 'id="update-conversation"' in html
+    assert 'id="update-subject-context"' in html
+    assert 'id="update-reply-workspace"' in html
+    assert '<script src="static/mobile-update-detail-nav.js"></script>' in html
+    assert "createMobileUpdateDetailNavigation({" in dashboard_js
+    assert "jumpToNewActivity:() => updateReadPosition.jump()" in dashboard_js
+    assert "mobileUpdateDetailNavigation.reset();" in dashboard_js
+    assert ".mobile-update-detail-nav" in css
+    assert "position:sticky" in css
+    assert "min-height:44px" in css
+    assert "env(safe-area-inset-top)" in css
+    assert "@media (min-width:601px)" in css
+    assert "BASE + 'static/mobile-update-detail-nav.js'" in service_worker
diff --git a/tests/test_service_worker.py b/tests/test_service_worker.py
index 142e3f1..c91151e 100644
--- a/tests/test_service_worker.py
+++ b/tests/test_service_worker.py
@@ -999,6 +999,7 @@ def test_install_precaches_complete_subpath_scoped_app_shell():
         "/dashboard/static/issue-filing-review.js",
         "/dashboard/static/issue-sheet.js",
         "/dashboard/static/mobile-issue-detail-nav.js",
+        "/dashboard/static/mobile-update-detail-nav.js",
         "/dashboard/static/checklist-conflict.js",
         "/dashboard/static/voice-transcript-store.js",
         "/dashboard/static/voice-issue-capture.js",