diff --git a/webui/src/components/settings/SettingsView.tsx b/webui/src/components/settings/SettingsView.tsx
index f41d9543..64f19b2d 100644
--- a/webui/src/components/settings/SettingsView.tsx
+++ b/webui/src/components/settings/SettingsView.tsx
@@ -112,8 +112,8 @@ import {
 import { notifyCliAppsChanged } from "@/lib/cli-app-events";
 import { copyTextToClipboard } from "@/lib/clipboard";
 import {
-  LOCAL_PREFS_STORAGE_KEY,
   readLocalPreferences,
+  writeLocalPreferences,
   type FileEditDisplayMode,
   type LocalActivityMode,
   type LocalDensity,
@@ -812,11 +812,7 @@ export function SettingsView({
   }, [activeSection, token]);
 
   useEffect(() => {
-    try {
-      window.localStorage.setItem(LOCAL_PREFS_STORAGE_KEY, JSON.stringify(localPrefs));
-    } catch {
-      // Browser-only preferences should never block settings.
-    }
+    writeLocalPreferences(localPrefs);
   }, [localPrefs]);
 
   useEffect(() => {
diff --git a/webui/src/hooks/useFileEditDisplayMode.ts b/webui/src/hooks/useFileEditDisplayMode.ts
index 6c340755..6effbbe2 100644
--- a/webui/src/hooks/useFileEditDisplayMode.ts
+++ b/webui/src/hooks/useFileEditDisplayMode.ts
@@ -1,6 +1,12 @@
 import { useEffect, useState } from "react";
 
-import { readLocalPreferences, type FileEditDisplayMode } from "@/lib/local-preferences";
+import {
+  LOCAL_PREFS_CHANGED_EVENT,
+  normalizeFileEditDisplayMode,
+  readLocalPreferences,
+  type FileEditDisplayMode,
+  type LocalPreferences,
+} from "@/lib/local-preferences";
 
 export function useFileEditDisplayMode(): FileEditDisplayMode {
   const [mode, setMode] = useState<FileEditDisplayMode>(() =>
@@ -9,11 +15,21 @@ export function useFileEditDisplayMode(): FileEditDisplayMode {
 
   useEffect(() => {
     const refresh = () => setMode(readLocalPreferences().fileEditDisplayMode);
+    const refreshFromLocalPreferenceEvent = (event: Event) => {
+      const detail = (event as CustomEvent<Partial<LocalPreferences> | undefined>).detail;
+      setMode(
+        detail
+          ? normalizeFileEditDisplayMode(detail.fileEditDisplayMode)
+          : readLocalPreferences().fileEditDisplayMode,
+      );
+    };
     window.addEventListener("storage", refresh);
     window.addEventListener("focus", refresh);
+    window.addEventListener(LOCAL_PREFS_CHANGED_EVENT, refreshFromLocalPreferenceEvent);
     return () => {
       window.removeEventListener("storage", refresh);
       window.removeEventListener("focus", refresh);
+      window.removeEventListener(LOCAL_PREFS_CHANGED_EVENT, refreshFromLocalPreferenceEvent);
     };
   }, []);
 
diff --git a/webui/src/lib/local-preferences.ts b/webui/src/lib/local-preferences.ts
index 7027857a..53e3c483 100644
--- a/webui/src/lib/local-preferences.ts
+++ b/webui/src/lib/local-preferences.ts
@@ -11,6 +11,7 @@ export interface LocalPreferences {
 }
 
 export const LOCAL_PREFS_STORAGE_KEY = "nanobot-webui.settings-preferences";
+export const LOCAL_PREFS_CHANGED_EVENT = "nanobot-webui.local-preferences-changed";
 
 export const DEFAULT_LOCAL_PREFS: LocalPreferences = {
   density: "comfortable",
@@ -40,3 +41,15 @@ export function readLocalPreferences(): LocalPreferences {
     return DEFAULT_LOCAL_PREFS;
   }
 }
+
+export function writeLocalPreferences(preferences: LocalPreferences): void {
+  try {
+    window.localStorage.setItem(LOCAL_PREFS_STORAGE_KEY, JSON.stringify(preferences));
+  } catch {
+    // Browser-only preferences should never block settings.
+  }
+  window.dispatchEvent(new CustomEvent<LocalPreferences>(
+    LOCAL_PREFS_CHANGED_EVENT,
+    { detail: preferences },
+  ));
+}
diff --git a/webui/src/tests/useFileEditDisplayMode.test.tsx b/webui/src/tests/useFileEditDisplayMode.test.tsx
new file mode 100644
index 00000000..74b4d7bb
--- /dev/null
+++ b/webui/src/tests/useFileEditDisplayMode.test.tsx
@@ -0,0 +1,37 @@
+import { act, render, screen, waitFor } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+
+import { useFileEditDisplayMode } from "@/hooks/useFileEditDisplayMode";
+import {
+  DEFAULT_LOCAL_PREFS,
+  LOCAL_PREFS_STORAGE_KEY,
+  writeLocalPreferences,
+} from "@/lib/local-preferences";
+
+function DisplayModeProbe() {
+  const mode = useFileEditDisplayMode();
+  return <div data-testid="mode">{mode}</div>;
+}
+
+describe("useFileEditDisplayMode", () => {
+  afterEach(() => {
+    localStorage.removeItem(LOCAL_PREFS_STORAGE_KEY);
+  });
+
+  it("updates when local preferences change in the same document", async () => {
+    render(<DisplayModeProbe />);
+
+    expect(screen.getByTestId("mode")).toHaveTextContent("summary");
+
+    act(() => {
+      writeLocalPreferences({
+        ...DEFAULT_LOCAL_PREFS,
+        fileEditDisplayMode: "diff",
+      });
+    });
+
+    await waitFor(() => {
+      expect(screen.getByTestId("mode")).toHaveTextContent("diff");
+    });
+  });
+});