diff --git a/components/calendar/calendar-sidebar-panel.tsx b/components/calendar/calendar-sidebar-panel.tsx
index 38dc2dccd..7dd844510 100644
--- a/components/calendar/calendar-sidebar-panel.tsx
+++ b/components/calendar/calendar-sidebar-panel.tsx
@@ -242,6 +242,10 @@ export function CalendarSidebarPanel({
         <button
           onClick={() => onToggleVisibility(cal.id)}
           onContextMenu={hasMenu ? (e) => openContextMenu(e, cal) : undefined}
+          data-testid="calendar-item"
+          data-calendar-name={cal.name}
+          data-account={cal.accountName ?? ''}
+          data-visible={isVisible}
           className={cn(
             "flex items-center gap-2 w-full px-1.5 py-1 rounded-md text-sm transition-colors duration-150",
             "hover:bg-muted"
diff --git a/components/settings/layout-settings.tsx b/components/settings/layout-settings.tsx
index 9cc305d1d..505b3e49f 100644
--- a/components/settings/layout-settings.tsx
+++ b/components/settings/layout-settings.tsx
@@ -354,6 +354,7 @@ export function LayoutSettings() {
         <ToggleSwitch
           checked={proInterface}
           onChange={(v) => updateSetting('proInterface', v)}
+          testId="setting-pro-interface"
         />
       </SettingItem>
     </SettingsSection>
diff --git a/components/settings/settings-app.tsx b/components/settings/settings-app.tsx
index 0fc31ba62..f3f527fdc 100644
--- a/components/settings/settings-app.tsx
+++ b/components/settings/settings-app.tsx
@@ -932,6 +932,7 @@ export function SettingsApp({ linkSegments }: SettingsAppProps = {}) {
                   return (
                     <div key={tab.id}>
                       <button
+                        data-testid={`settings-tab-${tab.id}`}
                         onClick={() => handleTabSelect(tab.id)}
                         className="w-full flex items-center justify-between px-5 py-3.5 text-sm text-foreground hover:bg-muted transition-colors duration-150"
                       >
@@ -1072,6 +1073,7 @@ export function SettingsApp({ linkSegments }: SettingsAppProps = {}) {
                   return (
                     <div key={tab.id}>
                       <button
+                        data-testid={`settings-tab-${tab.id}`}
                         onClick={() => handleTabSelect(tab.id)}
                         className={cn(
                           'w-full text-start px-3 py-2 rounded-md text-sm transition-colors duration-150 flex items-center gap-2.5',
diff --git a/components/settings/settings-section.tsx b/components/settings/settings-section.tsx
index c05d8fc74..9169d9a5a 100644
--- a/components/settings/settings-section.tsx
+++ b/components/settings/settings-section.tsx
@@ -67,14 +67,17 @@ interface ToggleSwitchProps {
   disabled?: boolean;
   /** Overrides the enclosing SettingItem label as the accessible name. */
   ariaLabel?: string;
+  /** Test hook (data-testid) for integration tests. */
+  testId?: string;
 }
 
-export function ToggleSwitch({ checked, onChange, disabled, ariaLabel }: ToggleSwitchProps) {
+export function ToggleSwitch({ checked, onChange, disabled, ariaLabel, testId }: ToggleSwitchProps) {
   const labelledBy = useContext(SettingLabelContext);
   return (
     <button
       type="button"
       role="switch"
+      data-testid={testId}
       aria-checked={checked}
       aria-label={ariaLabel}
       aria-labelledby={ariaLabel ? undefined : labelledBy}
diff --git a/integration/tests/11-multi-account-calendar.spec.ts b/integration/tests/11-multi-account-calendar.spec.ts
new file mode 100644
index 000000000..9e6bc259a
--- /dev/null
+++ b/integration/tests/11-multi-account-calendar.spec.ts
@@ -0,0 +1,89 @@
+import { test, expect, type Page } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import {
+  login,
+  addAccount,
+  enableProInterface,
+  openCalendar,
+  calendarItem,
+  calendarRows,
+} from './helpers/app';
+
+/**
+ * Multi-account calendar aggregation across an account switch.
+ *
+ * Regression for the cross-account id-namespacing bug: the Pro shell used to
+ * leave the ACTIVE account's calendar ids raw while namespacing the others, so
+ * switching accounts flipped the id form and reset the persisted calendar
+ * visibility selection (and broke subscription/mutation routing). All accounts
+ * are now namespaced consistently, so the selection survives a switch.
+ *
+ * Mail is unaffected (it keys per account + uses stable virtual ids), which is
+ * why the user saw this only in Calendar and Address Book.
+ */
+const { alice, bob } = ACCOUNTS;
+
+const switcher = (page: Page) => page.locator('[data-testid="account-switcher"]').first();
+
+/** Read an account's id from the (stable, standard-shell) switcher popover. */
+async function accountId(page: Page, email: string): Promise<string> {
+  await switcher(page).click();
+  const id = await page
+    .locator(`[data-testid="account-option"][data-account-email="${email}"]`)
+    .first()
+    .getAttribute('data-account-id');
+  await page.keyboard.press('Escape');
+  if (!id) throw new Error(`no account id for ${email}`);
+  return id;
+}
+
+/**
+ * Switch active account robustly in the Pro shell. The switcher popover is a
+ * portalled, repositioning element, so a normal click can race its layout;
+ * force the click and verify via the switcher's data-active-account-id.
+ */
+async function switchTo(page: Page, id: string): Promise<void> {
+  await switcher(page).click();
+  await page.locator(`[data-testid="account-option"][data-account-id="${id}"]`).first().click({ force: true });
+  await expect
+    .poll(async () => switcher(page).getAttribute('data-active-account-id'), { timeout: 30000 })
+    .toBe(id);
+}
+
+test.describe('Multi-account calendar', () => {
+  test('an account switch preserves the calendar visibility selection', async ({ page }) => {
+    // Both accounts in the standard shell (stable switcher), capture their ids,
+    // then flip on the Pro interface so the calendar aggregates across accounts.
+    await login(page, alice);
+    await addAccount(page, bob);
+    const bobId = await accountId(page, bob.email);
+    await enableProInterface(page);
+
+    await openCalendar(page);
+
+    // Both accounts' (default) calendars aggregate into one list with distinct
+    // names, and every row is initially visible.
+    const rows = await calendarRows(page);
+    expect(rows.length).toBeGreaterThanOrEqual(2);
+    expect(new Set(rows.map((r) => r.name)).size).toBeGreaterThanOrEqual(2);
+
+    // Deselect one calendar; remember it by name.
+    const target = rows.find((r) => r.visible);
+    expect(target, 'expected at least one visible calendar').toBeTruthy();
+    const name = target!.name;
+    await calendarItem(page, name).click();
+    await expect(calendarItem(page, name)).toHaveAttribute('data-visible', 'false');
+
+    // Switch the active account. This alone triggered the bug: the just-
+    // deselected calendar belonged to the previously-active account, whose ids
+    // flipped raw->namespaced on the switch, so the reconciler dropped the
+    // selection and reset visibility to "all".
+    await switchTo(page, bobId);
+    await openCalendar(page);
+
+    // The deselection survived the switch (the calendar is still aggregated in
+    // and still hidden), and the list is still the cross-account set.
+    await expect(calendarItem(page, name)).toHaveAttribute('data-visible', 'false');
+    expect((await calendarRows(page)).length).toBeGreaterThanOrEqual(2);
+  });
+});
diff --git a/integration/tests/helpers/app.ts b/integration/tests/helpers/app.ts
index f9859b960..34383dc75 100644
--- a/integration/tests/helpers/app.ts
+++ b/integration/tests/helpers/app.ts
@@ -378,3 +378,53 @@ export async function emailContextAction(page: Page, subject: string, testId: st
   }).toPass({ timeout: 15000 });
   await item.click();
 }
+
+// --- Calendar (multi-account) ---------------------------------------------
+
+/**
+ * Navigate to the calendar via the in-app nav (client-side routing) and wait
+ * for the sidebar calendar list. A hard `page.goto('/calendar')` redirects to
+ * login because the deep-link SSR path runs before client auth hydrates.
+ */
+export async function openCalendar(page: Page): Promise<void> {
+  await page.locator('a[href$="/calendar"]').first().click();
+  await page
+    .locator('[data-testid="calendar-item"]')
+    .first()
+    .waitFor({ state: 'visible', timeout: 30000 });
+}
+
+/** A sidebar calendar row by its (exact) name. */
+export function calendarItem(page: Page, name: string): Locator {
+  return page.locator(`[data-testid="calendar-item"][data-calendar-name="${name}"]`).first();
+}
+
+/** All sidebar calendars as {name, account, visible}. */
+export async function calendarRows(
+  page: Page,
+): Promise<Array<{ name: string; account: string; visible: boolean }>> {
+  return page.locator('[data-testid="calendar-item"]').evaluateAll((els) =>
+    els.map((e) => ({
+      name: e.getAttribute('data-calendar-name') || '',
+      account: e.getAttribute('data-account') || '',
+      visible: e.getAttribute('data-visible') === 'true',
+    })),
+  );
+}
+
+/**
+ * Turn on the Pro (multi-account) interface via the settings UI, so the
+ * calendar/contacts aggregate across accounts. Done through the real toggle
+ * (not a reload/seed) because the app doesn't survive a hard reload in tests
+ * and seeding proInterface before login destabilises the add-account popover.
+ */
+export async function enableProInterface(page: Page): Promise<void> {
+  await page.locator('a[href$="/settings"]').first().click();
+  await page.locator('[data-testid="settings-tab-layout"]').first().click();
+  const toggle = page.locator('[data-testid="setting-pro-interface"]').first();
+  await toggle.waitFor({ state: 'visible', timeout: 30000 });
+  if ((await toggle.getAttribute('aria-checked')) !== 'true') {
+    await toggle.click();
+    await expect(toggle).toHaveAttribute('aria-checked', 'true');
+  }
+}