diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index 3de801e95..c2fd793f1 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -4655,7 +4655,7 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
           {/* Email List - full width on mobile, fixed width/height on tablet/desktop */}
           <div
             className={cn(
-              "relative flex flex-col bg-background",
+              "relative flex flex-col bg-background min-w-0",
               isHorizontalMailLayout
                 ? (shouldHideHorizontalViewerPane ? "md:w-full md:min-h-0" : "md:w-full md:h-auto")
                 : "h-full border-e border-border",
diff --git a/integration/README.md b/integration/README.md
index a4d68c237..6ccffd973 100644
--- a/integration/README.md
+++ b/integration/README.md
@@ -158,6 +158,7 @@ because the UI behaviour is currently incomplete. Worth a look:
 | `IT_WEBMAIL_URL`| `http://localhost:3000` | Webmail origin                                        |
 | `IT_JMAP_URL`   | `http://localhost:8025` | Stalwart JMAP/admin base URL                          |
 | `IT_SMTP_PORT`  | `1025`             | Stalwart submission port                                   |
+| `IT_ACCOUNT_USERS` | `alice,bob,carol` | Local parts of the three test accounts, for a stack with other users |
 | `IT_VIDEO`      | `retain-on-failure`| Video capture: `on` records a `.webm` for **every** test; also `off` / `on-first-retry`. Videos land at `integration/test-results/<test>/video.webm` |
 
 Record videos for a whole run (passing tests included):
diff --git a/integration/tests/16-phone-layout.spec.ts b/integration/tests/16-phone-layout.spec.ts
new file mode 100644
index 000000000..46b8bd4de
--- /dev/null
+++ b/integration/tests/16-phone-layout.spec.ts
@@ -0,0 +1,63 @@
+import { test, expect, devices } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import { sendMail } from './helpers/smtp';
+import { JmapClient } from './helpers/jmap';
+import { login, emailItem, expectEmailVisible } from './helpers/app';
+
+/**
+ * On a phone the message list is the whole screen. The selection toolbar
+ * above it stays in the layout while collapsed, and its row of nowrap
+ * buttons is wider than a phone. The list pane used to grow to that width
+ * (a flex item with min-width:auto), which pushed the header's buttons,
+ * every row's date and the subject's ellipsis off the right edge of the
+ * screen, with nothing selected. The toolbar then measured itself as wide
+ * and kept its labels too.
+ */
+const alice = ACCOUNTS.alice;
+const subject = `IT phone-layout ${Date.now()}`;
+
+test.use({ ...devices['Pixel 7'] });
+
+/** Fails when `locator` has no box or any part of it is off the screen's sides. */
+async function expectOnScreen(locator: import('@playwright/test').Locator, width: number): Promise<void> {
+  const box = await locator.boundingBox();
+  expect(box).not.toBeNull();
+  expect(box!.x).toBeGreaterThanOrEqual(0);
+  expect(box!.x + box!.width).toBeLessThanOrEqual(width + 0.5);
+}
+
+test.describe('phone layout', () => {
+  test.beforeEach(async () => {
+    const jmap = await JmapClient.connect(alice.email, alice.password);
+    await jmap.reset();
+    await sendMail({ from: alice.email, authPass: alice.password, to: alice.email, subject, body: 'x' });
+    await jmap.waitForEmail(subject);
+  });
+
+  test('the message list is never wider than the screen', async ({ page }) => {
+    await login(page, alice);
+    await expectEmailVisible(page, subject);
+    const { width } = page.viewportSize()!;
+
+    expect(await page.evaluate(() => document.documentElement.scrollWidth)).toBeLessThanOrEqual(width);
+    await expectOnScreen(page.locator('[data-mobile-mail-header]'), width);
+    // The account button is the header's last control.
+    await expectOnScreen(page.locator('[data-testid="account-switcher"]').first(), width);
+    await expectOnScreen(emailItem(page, subject).first(), width);
+  });
+
+  test('the selection toolbar compacts so every button is on screen', async ({ page }) => {
+    await login(page, alice);
+    await expectEmailVisible(page, subject);
+    const { width } = page.viewportSize()!;
+
+    await emailItem(page, subject).first().getByRole('checkbox').click();
+    // The toolbar only drops its labels when it measures narrower than a desktop list.
+    const toolbar = page.locator('[data-batch-narrow]');
+    await expect(toolbar).toBeVisible();
+    await expectOnScreen(toolbar, width);
+    for (const button of await toolbar.getByRole('button').all()) {
+      await expectOnScreen(button, width);
+    }
+  });
+});
diff --git a/integration/tests/helpers/config.ts b/integration/tests/helpers/config.ts
index b97998b41..90eec36fb 100644
--- a/integration/tests/helpers/config.ts
+++ b/integration/tests/helpers/config.ts
@@ -35,11 +35,16 @@ function acct(user: string): TestAccount {
   return { user, email: `${user}@${DOMAIN}`, password: ACCOUNT_PASSWORD };
 }
 
-/** The mailboxes provisioned by the Stalwart bootstrap plan. */
+/**
+ * The mailboxes provisioned by the Stalwart bootstrap plan. IT_ACCOUNT_USERS
+ * renames the three local parts (in this order) for a stack with other users,
+ * such as the shared test servers.
+ */
+const USERS = (process.env.IT_ACCOUNT_USERS ?? 'alice,bob,carol').split(',');
 export const ACCOUNTS = {
-  alice: acct('alice'),
-  bob: acct('bob'),
-  carol: acct('carol'),
+  alice: acct(USERS[0]),
+  bob: acct(USERS[1]),
+  carol: acct(USERS[2]),
 } as const;
 
 export type AccountKey = keyof typeof ACCOUNTS;