diff --git a/src/core/services/yomitan-extension-copy.ts b/src/core/services/yomitan-extension-copy.ts
new file mode 100644
index 0000000..8c90f32
--- /dev/null
+++ b/src/core/services/yomitan-extension-copy.ts
@@ -0,0 +1,53 @@
+import * as fs from 'fs';
+import * as path from 'path';
+
+const YOMITAN_SYNC_SCRIPT_PATHS = [
+  path.join('js', 'app', 'popup.js'),
+  path.join('js', 'display', 'popup-main.js'),
+  path.join('js', 'display', 'display.js'),
+  path.join('js', 'display', 'display-audio.js'),
+];
+
+function readManifestVersion(manifestPath: string): string | null {
+  try {
+    const parsed = JSON.parse(fs.readFileSync(manifestPath, 'utf-8')) as { version?: unknown };
+    return typeof parsed.version === 'string' ? parsed.version : null;
+  } catch {
+    return null;
+  }
+}
+
+function areFilesEqual(sourcePath: string, targetPath: string): boolean {
+  if (!fs.existsSync(sourcePath) || !fs.existsSync(targetPath)) return false;
+  try {
+    return fs.readFileSync(sourcePath).equals(fs.readFileSync(targetPath));
+  } catch {
+    return false;
+  }
+}
+
+export function shouldCopyYomitanExtension(sourceDir: string, targetDir: string): boolean {
+  if (!fs.existsSync(targetDir)) {
+    return true;
+  }
+
+  const sourceManifest = path.join(sourceDir, 'manifest.json');
+  const targetManifest = path.join(targetDir, 'manifest.json');
+  if (!fs.existsSync(sourceManifest) || !fs.existsSync(targetManifest)) {
+    return true;
+  }
+
+  const sourceVersion = readManifestVersion(sourceManifest);
+  const targetVersion = readManifestVersion(targetManifest);
+  if (sourceVersion === null || targetVersion === null || sourceVersion !== targetVersion) {
+    return true;
+  }
+
+  for (const relativePath of YOMITAN_SYNC_SCRIPT_PATHS) {
+    if (!areFilesEqual(path.join(sourceDir, relativePath), path.join(targetDir, relativePath))) {
+      return true;
+    }
+  }
+
+  return false;
+}
diff --git a/src/core/services/yomitan-extension-loader.test.ts b/src/core/services/yomitan-extension-loader.test.ts
new file mode 100644
index 0000000..aaf645e
--- /dev/null
+++ b/src/core/services/yomitan-extension-loader.test.ts
@@ -0,0 +1,52 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+
+import { shouldCopyYomitanExtension } from './yomitan-extension-copy';
+
+function writeFile(filePath: string, content: string): void {
+  fs.mkdirSync(path.dirname(filePath), { recursive: true });
+  fs.writeFileSync(filePath, content, 'utf-8');
+}
+
+test('shouldCopyYomitanExtension detects popup runtime script drift', () => {
+  const tempRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'yomitan-copy-test-'));
+  const sourceDir = path.join(tempRoot, 'source');
+  const targetDir = path.join(tempRoot, 'target');
+
+  writeFile(path.join(sourceDir, 'manifest.json'), JSON.stringify({ version: '1.0.0' }));
+  writeFile(path.join(targetDir, 'manifest.json'), JSON.stringify({ version: '1.0.0' }));
+
+  writeFile(path.join(sourceDir, 'js', 'app', 'popup.js'), 'same-popup-script');
+  writeFile(path.join(targetDir, 'js', 'app', 'popup.js'), 'same-popup-script');
+
+  writeFile(path.join(sourceDir, 'js', 'display', 'popup-main.js'), 'source-popup-main');
+  writeFile(path.join(targetDir, 'js', 'display', 'popup-main.js'), 'target-popup-main');
+
+  assert.equal(shouldCopyYomitanExtension(sourceDir, targetDir), true);
+});
+
+test('shouldCopyYomitanExtension skips copy when versions and watched scripts match', () => {
+  const tempRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'yomitan-copy-test-'));
+  const sourceDir = path.join(tempRoot, 'source');
+  const targetDir = path.join(tempRoot, 'target');
+
+  writeFile(path.join(sourceDir, 'manifest.json'), JSON.stringify({ version: '1.0.0' }));
+  writeFile(path.join(targetDir, 'manifest.json'), JSON.stringify({ version: '1.0.0' }));
+
+  writeFile(path.join(sourceDir, 'js', 'app', 'popup.js'), 'same-popup-script');
+  writeFile(path.join(targetDir, 'js', 'app', 'popup.js'), 'same-popup-script');
+
+  writeFile(path.join(sourceDir, 'js', 'display', 'popup-main.js'), 'same-popup-main');
+  writeFile(path.join(targetDir, 'js', 'display', 'popup-main.js'), 'same-popup-main');
+
+  writeFile(path.join(sourceDir, 'js', 'display', 'display.js'), 'same-display');
+  writeFile(path.join(targetDir, 'js', 'display', 'display.js'), 'same-display');
+
+  writeFile(path.join(sourceDir, 'js', 'display', 'display-audio.js'), 'same-display-audio');
+  writeFile(path.join(targetDir, 'js', 'display', 'display-audio.js'), 'same-display-audio');
+
+  assert.equal(shouldCopyYomitanExtension(sourceDir, targetDir), false);
+});
diff --git a/src/core/services/yomitan-extension-loader.ts b/src/core/services/yomitan-extension-loader.ts
index 59b6fd5..fcde4dc 100644
--- a/src/core/services/yomitan-extension-loader.ts
+++ b/src/core/services/yomitan-extension-loader.ts
@@ -2,6 +2,7 @@ import { BrowserWindow, Extension, session } from 'electron';
 import * as fs from 'fs';
 import * as path from 'path';
 import { createLogger } from '../../logger';
+import { shouldCopyYomitanExtension } from './yomitan-extension-copy';
 
 const logger = createLogger('main:yomitan-extension-loader');
 
@@ -22,27 +23,7 @@ function ensureExtensionCopy(sourceDir: string, userDataPath: string): string {
   const extensionsRoot = path.join(userDataPath, 'extensions');
   const targetDir = path.join(extensionsRoot, 'yomitan');
 
-  const sourceManifest = path.join(sourceDir, 'manifest.json');
-  const targetManifest = path.join(targetDir, 'manifest.json');
-
-  let shouldCopy = !fs.existsSync(targetDir);
-  if (!shouldCopy && fs.existsSync(sourceManifest) && fs.existsSync(targetManifest)) {
-    try {
-      const sourceVersion = (
-        JSON.parse(fs.readFileSync(sourceManifest, 'utf-8')) as {
-          version: string;
-        }
-      ).version;
-      const targetVersion = (
-        JSON.parse(fs.readFileSync(targetManifest, 'utf-8')) as {
-          version: string;
-        }
-      ).version;
-      shouldCopy = sourceVersion !== targetVersion;
-    } catch {
-      shouldCopy = true;
-    }
-  }
+  const shouldCopy = shouldCopyYomitanExtension(sourceDir, targetDir);
 
   if (shouldCopy) {
     fs.mkdirSync(extensionsRoot, { recursive: true });