AG-37909 do not inject content scripts between extension wakeups

Author: maximtop

Extension/src/background/app/app-mv2.ts

@@ -140,18 +140,22 @@ export class App {
         await UiApi.init();
 
         /**
-         * When the extension is enabled, disabled and re-enabled during the user session,
-         * content scripts will be loaded multiple times in each open tab.
-         * If statistics collection is enabled, the content script will initialize cssHitCounter.
-         * Multiple cssHitCounters in the same page context will conflict with each other,
-         * with a high probability of breaking the page.
-         * To avoid this bug, we don't inject content scripts into open tabs during initialization
-         * when stats collection is enabled.
+         * Injects content scripts into already open tabs.
+         *
+         * Skips injection when:
+         * - Statistics collection is enabled.
+         * - Content scripts have already been injected in the current session.
+         *
+         * This prevents conflicts from multiple `cssHitCounters` and avoids unnecessary injections.
          */
-        if (SettingsApi.getSetting(SettingOption.DisableCollectHits)) {
-            // inject content scripts into opened tabs
+        if (
+            SettingsApi.getSetting(SettingOption.DisableCollectHits)
+            && !await ContentScriptInjector.isInjected()
+        ) {
             await ContentScriptInjector.init();
+            await ContentScriptInjector.setInjected();
         }
+
         /**
          * Initializes Filters data:
          * - Loads app i18n metadata and caches it in i18n-metadata storage

Extension/src/background/app/app-mv3.ts

@@ -153,7 +153,7 @@ export class App {
 
         await rulesLimitsService.init();
 
-        // TODO mv3 uses other way to inject scripts. AG-33507
+        // TODO inject content scripts to tabs open before installation AG-33507
         // /**
         //  * When the extension is enabled, disabled and re-enabled during the user session,
         //  * content scripts will be loaded multiple times in each open tab.

Extension/src/background/content-script-injector.ts

@@ -17,7 +17,7 @@
  */
 
 import { getHostname } from 'tldts';
-import { type Tabs } from 'webextension-polyfill';
+import browser, { type Tabs } from 'webextension-polyfill';
 
 import { isHttpRequest } from 'tswebextension';
 
@@ -39,6 +39,11 @@ import { createPromiseWithTimeout } from './utils/timeouts';
  * Helper class for injecting content script into tabs, opened before extension initialization.
  */
 export class ContentScriptInjector {
+    /**
+     * Key used to store the injected flag in session storage.
+     */
+    private static INJECTED_KEY = 'content_script_injected';
+
     private static INJECTION_LIMIT_MS = 1000;
 
     /**
@@ -202,4 +207,36 @@ export class ContentScriptInjector {
 
         return true;
     }
+
+    /**
+     * Sets the injected flag in session storage.
+     * This method updates the session storage to indicate that content scripts have been injected.
+     */
+    public static async setInjected(): Promise<void> {
+        try {
+            await browser.storage.session.set({ [ContentScriptInjector.INJECTED_KEY]: true });
+        } catch (e) {
+            logger.error('Cannot set injected flag in session storage', e);
+        }
+    }
+
+    /**
+     * Checks if content scripts have been injected.
+     * Uses session storage since it is faster than sending a message to the content script.
+     * As of November 25, 2025, Firefox v132.0.2 takes 1 second to send a message,
+     * whereas reading from the session storage takes only 1 ms.
+     *
+     * @returns True if content scripts were injected; otherwise, false.
+     */
+    public static async isInjected(): Promise<boolean> {
+        let isInjected = false;
+        try {
+            const result = await browser.storage.session.get(ContentScriptInjector.INJECTED_KEY);
+            isInjected = result[ContentScriptInjector.INJECTED_KEY] === true;
+        } catch (e) {
+            logger.error('Cannot get injected flag from session storage', e);
+        }
+
+        return isInjected;
+    }
 }