diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index b03b2e9..58c26ae 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,7 +1,14 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
-import { api, HttpError, setUnauthorizedHandler, type FeedFilters } from "./lib/api";
+import {
+  api,
+  getActiveAccount,
+  HttpError,
+  setActiveAccount,
+  setUnauthorizedHandler,
+  type FeedFilters,
+} from "./lib/api";
 import { setLanguage, isSupported, type LangCode } from "./i18n";
 import {
   applyTheme,
@@ -367,6 +374,15 @@ export default function App() {
     refetchInterval: (query) => (query.state.data ? 60_000 : false),
   });
 
+  // Pin this tab to whatever account it first loaded as. Without this, a tab that never picked
+  // an account (it was riding the session's default) would silently swap identity when ANOTHER
+  // tab changes the default — e.g. that other tab adds/switches an account. Once pinned, this
+  // tab always sends its own account header, so cross-tab changes can't reach it. An explicit
+  // switch on THIS tab sets the override itself (so this no-ops then).
+  useEffect(() => {
+    if (meQuery.data && getActiveAccount() == null) setActiveAccount(meQuery.data.id);
+  }, [meQuery.data?.id]);
+
   // Once signed in, drop any leftover query string from the pre-login flow (e.g.
   // ?access=requested, ?verify, ?reset). The logged-in app reads nothing from the URL, so the
   // address bar stays clean. Gated on auth so the logged-out Welcome page still reads its params.
diff --git a/frontend/src/components/NavSidebar.tsx b/frontend/src/components/NavSidebar.tsx
index 578a7ec..db5dde3 100644
--- a/frontend/src/components/NavSidebar.tsx
+++ b/frontend/src/components/NavSidebar.tsx
@@ -402,6 +402,10 @@ export default function NavSidebar({
                 <div className="mt-2 pt-2 border-t border-border flex flex-col">
                   <button
                     onClick={() => {
+                      // Adding an account switches THIS tab to it: drop the tab's pin so that on
+                      // return from Google it adopts the freshly-added account (the new default)
+                      // and pins that. Other tabs keep their own pinned account.
+                      clearActiveAccount();
                       window.location.href = "/auth/login";
                     }}
                     className="w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"