diff --git a/docs/flasher/index.html b/docs/flasher/index.html
index cfcb8215..67e4c3ad 100644
--- a/docs/flasher/index.html
+++ b/docs/flasher/index.html
@@ -462,6 +462,10 @@
                 setMapStatus('This browser does not provide writable directory access. Use Chrome, Chromium, or Edge; Brave currently disables this API.', 'error');
                 return;
             }
+            if (typeof navigator.locks?.request !== 'function') {
+                setMapStatus('This browser does not provide the cross-tab locking required for safe map installation. Use a current Chrome, Chromium, or Edge release.', 'error');
+                return;
+            }
             mapInstallBtn.disabled = true;
             try {
                 const rootDirectory = await window.showDirectoryPicker({mode: 'readwrite'});
diff --git a/docs/flasher/js/map-installer.js b/docs/flasher/js/map-installer.js
index dc7003a3..d4775fb2 100644
--- a/docs/flasher/js/map-installer.js
+++ b/docs/flasher/js/map-installer.js
@@ -19,7 +19,7 @@ const textDecoder = new TextDecoder('utf-8', {fatal: true});
 const textEncoder = new TextEncoder();
 const installTails = new WeakMap();
 
-async function withInstallLock(rootDirectory, operation) {
+async function withInProcessInstallLock(rootDirectory, operation) {
   const previous = installTails.get(rootDirectory) || Promise.resolve();
   let release;
   const gate = new Promise(resolve => { release = resolve; });
@@ -33,6 +33,28 @@ async function withInstallLock(rootDirectory, operation) {
   }
 }
 
+async function withInstallLock(rootDirectory, operation) {
+  const lockManager = globalThis.navigator?.locks;
+  if (typeof lockManager?.request === 'function') {
+    if (typeof rootDirectory?.name !== 'string' || rootDirectory.name.length === 0) {
+      fail('Selected SD root has no stable directory name');
+    }
+    // Web Locks are shared by every same-origin installer tab. Distinct
+    // FileSystemDirectoryHandle objects for the same root have the same name;
+    // equal names on unrelated roots merely serialize harmlessly.
+    return lockManager.request(
+      `pyxis-map-installer:${rootDirectory.name}`,
+      {mode:'exclusive'},
+      operation,
+    );
+  }
+  if (typeof globalThis.window?.showDirectoryPicker === 'function') {
+    fail('This browser lacks the cross-tab locking required for safe map installation');
+  }
+  // Headless contract tests do not expose browser lock primitives.
+  return withInProcessInstallLock(rootDirectory, operation);
+}
+
 export class MapInstallerError extends Error {}
 function fail(message) { throw new MapInstallerError(message); }
 function u16(view, offset) { return view.getUint16(offset, true); }
diff --git a/tests/build_scripts/test_map_web_installer_contract.py b/tests/build_scripts/test_map_web_installer_contract.py
index 81e9ce89..adac9bb4 100644
--- a/tests/build_scripts/test_map_web_installer_contract.py
+++ b/tests/build_scripts/test_map_web_installer_contract.py
@@ -20,6 +20,8 @@ def test_map_installer_ui_is_local_file_to_sd_and_offline_only() -> None:
     assert "mapSetId: 'osm-bright'" in source
     assert 'newest pack takes priority' in source
     assert "showDirectoryPicker" in source
+    assert "navigator.locks?.request" in source
+    assert "cross-tab locking required for safe map installation" in source
     assert "./js/map-installer.js" in source
     assert "Coalition MUI OSM Bright user download" in source
     assert "Map data (c) OpenStreetMap contributors" in source
diff --git a/tests/web/test_map_installer.mjs b/tests/web/test_map_installer.mjs
index 476f58f0..7b7e1a1b 100644
--- a/tests/web/test_map_installer.mjs
+++ b/tests/web/test_map_installer.mjs
@@ -141,6 +141,32 @@ class MemoryDirectoryHandle {
   }
 }
 
+class DirectoryHandleAlias {
+  constructor(target) { this.target = target; this.name = target.name; this.kind = 'directory'; }
+  async getDirectoryHandle(...args) { return this.target.getDirectoryHandle(...args); }
+  async getFileHandle(...args) { return this.target.getFileHandle(...args); }
+  async *entries() { yield* this.target.entries(); }
+  async removeEntry(...args) { return this.target.removeEntry(...args); }
+  async isSameEntry(other) { return this.target === (other?.target || other); }
+}
+
+class MemoryLockManager {
+  constructor() { this.tails = new Map(); }
+  async request(name, _options, operation) {
+    const previous = this.tails.get(name) || Promise.resolve();
+    let release;
+    const gate = new Promise(resolve => { release = resolve; });
+    const tail = previous.then(() => gate);
+    this.tails.set(name, tail);
+    await previous;
+    try { return await operation(); }
+    finally {
+      release();
+      if (this.tails.get(name) === tail) this.tails.delete(name);
+    }
+  }
+}
+
 async function child(root, path) {
   let current = root;
   for (const part of path.split('/')) current = current.children.get(part);
@@ -322,6 +348,35 @@ test('concurrent installs on one selected root are serialized', async () => {
   assert.deepEqual(new Set(newest.packs.map(pack => pack.packId)), new Set(['first-pack', 'second-pack']));
 });
 
+test('distinct handles for one selected root are serialized across tabs', async () => {
+  const archive = storedZip([['2/1/1.png', PNG]]);
+  const root = new MemoryDirectoryHandle('sd-card');
+  const firstHandle = new DirectoryHandleAlias(root);
+  const secondHandle = new DirectoryHandleAlias(root);
+  const previousNavigator = Object.getOwnPropertyDescriptor(globalThis, 'navigator');
+  Object.defineProperty(globalThis, 'navigator', {
+    configurable:true,
+    value:{locks:new MemoryLockManager()},
+  });
+  try {
+    await Promise.all([
+      installMuiZip({archive, rootDirectory:firstHandle, metadata:{...metadata, packId:'tab-one', name:'Tab One'}}),
+      installMuiZip({archive, rootDirectory:secondHandle, metadata:{...metadata, packId:'tab-two', name:'Tab Two'}}),
+    ]);
+  } finally {
+    if (previousNavigator) Object.defineProperty(globalThis, 'navigator', previousNavigator);
+    else delete globalThis.navigator;
+  }
+  const pyxis = root.children.get('pyxis-map');
+  const slots = ['active-pack.0', 'active-pack.1']
+    .map(name => pyxis.children.get(name))
+    .filter(Boolean)
+    .map(handle => decodeActiveSelection(handle.bytes));
+  const newest = slots.sort((left, right) => right.generation - left.generation)[0];
+  assert.equal(newest.generation, 2);
+  assert.deepEqual(new Set(newest.packs.map(pack => pack.packId)), new Set(['tab-one', 'tab-two']));
+});
+
 test('an empty directory created during destination creation is never removed', async () => {
   const archive = storedZip([['2/1/1.png', PNG]]);
   const root = new MemoryDirectoryHandle();