fix: let the user cancel a sign-in that hangs

src/api/jmap-client.ts

@@ -279,11 +279,13 @@ export class JMAPClient {
     username: string,
     password: string,
     totp?: string,
+    /** Cancels the token requests of a TOTP sign-in (the sign-in screen's Cancel). */
+    signal?: AbortSignal,
   ): Promise<JMAPSession> {
     const baseUrl = serverUrl.replace(/\/+$/, '');
     const epoch = this.detachSession();
     if (totp) {
-      return this.connectWithTotp(baseUrl, username, password, totp, epoch);
+      return this.connectWithTotp(baseUrl, username, password, totp, epoch, signal);
     }
     const credentials: StoredCredentials = { serverUrl: baseUrl, username, password };
     this.credentials = credentials;
@@ -314,11 +316,12 @@ export class JMAPClient {
     password: string,
     totp: string,
     epoch: number,
+    signal?: AbortSignal,
   ): Promise<JMAPSession> {
     const { exchangePasswordForTokens, LoginEndpointMissingError } = await import('../lib/totp-login');
     let tokens: OAuthTokens;
     try {
-      tokens = await exchangePasswordForTokens(baseUrl, username, password, totp);
+      tokens = await exchangePasswordForTokens(baseUrl, username, password, totp, { signal });
     } catch (err) {
       if (err instanceof LoginEndpointMissingError) {
         this.assertCurrentSession(epoch);

src/lib/__tests__/server-discovery.test.ts

@@ -196,4 +196,16 @@ describe('discoverServerForEmail', () => {
     await expect(discoverServerForEmail('ada')).resolves.toBeNull();
     expect(mockSecureFetch).not.toHaveBeenCalled();
   });
+
+  it('stops searching as soon as the sign-in is cancelled (U-133)', async () => {
+    // Every host hangs; only the cancel can end the search before the deadline.
+    mockSecureFetch.mockImplementation(() => new Promise(() => undefined));
+    const controller = new AbortController();
+    const started = Date.now();
+    const search = discoverServerForEmail('ada@example.com', { timeoutMs: 60_000, signal: controller.signal });
+    setTimeout(() => controller.abort(), 20);
+    await expect(search).resolves.toBeNull();
+    expect(Date.now() - started).toBeLessThan(5_000);
+    expect(mockSecureFetch.mock.calls[0][1].signal.aborted).toBe(true);
+  });
 });

src/lib/__tests__/totp-login.test.ts

@@ -101,6 +101,23 @@ describe('exchangePasswordForTokens', () => {
     expect(calls.some((c) => c.url.endsWith('/auth/register'))).toBe(false);
   });
 
+  it('cancels the client check and the login requests with the sign-in screen (U-133)', async () => {
+    const signals: Array<AbortSignal | undefined> = [];
+    fetchMock.mockImplementation((_url: string, init?: RequestInit) => {
+      signals.push(init?.signal ?? undefined);
+      return new Promise((_resolve, reject) => {
+        init?.signal?.addEventListener('abort', () => reject(new TypeError('Aborted')));
+      });
+    });
+    const controller = new AbortController();
+    const pending = exchangePasswordForTokens(BASE, 'u@example.org', 'pw', '123456', { signal: controller.signal });
+    await vi.waitFor(() => expect(signals.length).toBe(1));
+    controller.abort();
+    // A cancel, not "server unreachable".
+    await expect(pending).rejects.toMatchObject({ name: 'SignInCancelledError' });
+    expect(signals[0]?.aborted).toBe(true);
+  });
+
   it('registers its own client on Stalwart 1.0 and signs in with it', async () => {
     const calls = fakeServer('1.0');
     const tokens = await exchangePasswordForTokens(BASE, 'u@example.org', 'pw', '123456');

src/lib/server-discovery.ts

@@ -8,6 +8,7 @@
 // slow or hostile network must never be worse than typing it in.
 
 import { secureFetch } from './client-cert';
+import { signalWithTimeout } from './sign-in-abort';
 
 export const DISCOVERY_TIMEOUT_MS = 2500;
 
@@ -96,18 +97,21 @@ export function serverCandidates(domain: string): string[] {
   return [`https://${domain}`, `https://mail.${domain}`, `https://webmail.${domain}`];
 }
 
-function withTimeout<T>(promise: Promise<T>, ms: number, fallback: T): Promise<T> {
+function withTimeout<T>(promise: Promise<T>, ms: number, fallback: T, signal?: AbortSignal): Promise<T> {
   return new Promise<T>((resolve) => {
-    const timer = setTimeout(() => resolve(fallback), ms);
-    promise
-      .then((value) => {
-        clearTimeout(timer);
-        resolve(value);
-      })
-      .catch(() => {
-        clearTimeout(timer);
-        resolve(fallback);
-      });
+    if (signal?.aborted) {
+      resolve(fallback);
+      return;
+    }
+    const finish = (value: T) => {
+      clearTimeout(timer);
+      signal?.removeEventListener('abort', onAbort);
+      resolve(value);
+    };
+    const onAbort = () => finish(fallback);
+    const timer = setTimeout(() => finish(fallback), ms);
+    signal?.addEventListener('abort', onAbort, { once: true });
+    promise.then(finish).catch(() => finish(fallback));
   });
 }
 
@@ -128,15 +132,16 @@ function looksLikeSession(body: unknown): boolean {
 export async function probeJmapServer(
   baseUrl: string,
   timeoutMs: number = DISCOVERY_TIMEOUT_MS,
+  signal?: AbortSignal,
 ): Promise<boolean> {
-  const controller = typeof AbortController !== 'undefined' ? new AbortController() : null;
-  const abortTimer = setTimeout(() => controller?.abort(), timeoutMs);
+  // Gives up at the deadline, or when the user cancels the sign-in.
+  const deadline = signalWithTimeout(signal, timeoutMs);
 
   const request = (async () => {
     try {
       const response = await secureFetch(`${baseUrl}/.well-known/jmap`, {
         headers: { Accept: 'application/json' },
-        signal: controller?.signal,
+        signal: deadline.signal,
       });
       if (response.status === 401) return true;
       if (response.status !== 200) return false;
@@ -149,9 +154,9 @@ export async function probeJmapServer(
   try {
     // The abort signal doesn't reach the native client-certificate path, so
     // race the whole thing rather than trusting the request to give up.
-    return await withTimeout(request, timeoutMs, false);
+    return await withTimeout(request, timeoutMs, false, signal);
   } finally {
-    clearTimeout(abortTimer);
+    deadline.done();
   }
 }
 
@@ -159,6 +164,8 @@ export interface DiscoveryOptions {
   /** Servers of already-registered accounts, trusted without a probe. */
   knownServerUrls?: string[];
   timeoutMs?: number;
+  /** The sign-in screen's Cancel: discovery stops and finds nothing. */
+  signal?: AbortSignal;
 }
 
 /**
@@ -170,7 +177,7 @@ export async function discoverServerForEmail(
   options: DiscoveryOptions = {},
 ): Promise<string | null> {
   const domain = emailDomain(email);
-  if (!domain) return null;
+  if (!domain || options.signal?.aborted) return null;
 
   // A server we're already signed in to for this domain needs no probe, and
   // covers the common case of adding a second account on the same host.
@@ -186,8 +193,9 @@ export async function discoverServerForEmail(
   const timeoutMs = options.timeoutMs ?? DISCOVERY_TIMEOUT_MS;
   const candidates = serverCandidates(domain);
   const results = await Promise.all(
-    candidates.map(async (candidate) => ((await probeJmapServer(candidate, timeoutMs)) ? candidate : null)),
+    candidates.map(async (candidate) => ((await probeJmapServer(candidate, timeoutMs, options.signal)) ? candidate : null)),
   );
+  if (options.signal?.aborted) return null;
 
   // `find` keeps candidate order, so a bare domain wins over `mail.` even when
   // both answer.

src/lib/sign-in-abort.ts

@@ -0,0 +1,73 @@
+// Cancelling a sign-in (U-133). The sign-in screen used to show "Signing in"
+// with no way out until every request gave up on its own, and Android Back
+// was swallowed meanwhile. The screen now holds an AbortController per
+// attempt; its signal goes into discovery, the client check, the /api/auth
+// and token requests, and the auth store, which drops a sign-in whose signal
+// fired before it registered the account.
+
+export class SignInCancelledError extends Error {
+  constructor() {
+    super('Sign-in cancelled');
+    this.name = 'SignInCancelledError';
+  }
+}
+
+export function isSignInCancelled(err: unknown): boolean {
+  return err instanceof Error && err.name === 'SignInCancelledError';
+}
+
+export function throwIfCancelled(signal: AbortSignal | undefined): void {
+  if (signal?.aborted) throw new SignInCancelledError();
+}
+
+/**
+ * `promise`, or a `SignInCancelledError` as soon as `signal` fires. The work
+ * behind the promise goes on; callers that need to undo what it does once it
+ * finishes keep their own reference to it.
+ */
+export function abortable<T>(promise: Promise<T>, signal: AbortSignal | undefined): Promise<T> {
+  if (!signal) return promise;
+  if (signal.aborted) {
+    promise.catch(() => undefined);
+    return Promise.reject(new SignInCancelledError());
+  }
+  return new Promise<T>((resolve, reject) => {
+    const onAbort = () => reject(new SignInCancelledError());
+    signal.addEventListener('abort', onAbort, { once: true });
+    promise.then(
+      (value) => {
+        signal.removeEventListener('abort', onAbort);
+        resolve(value);
+      },
+      (err) => {
+        signal.removeEventListener('abort', onAbort);
+        reject(signal.aborted ? new SignInCancelledError() : err);
+      },
+    );
+  });
+}
+
+/**
+ * One signal that fires when `signal` does or after `ms`, whichever comes
+ * first (AbortSignal.any and AbortSignal.timeout aren't in every engine the
+ * app runs on). Call `done` when the request has finished.
+ */
+export function signalWithTimeout(
+  signal: AbortSignal | undefined,
+  ms?: number,
+): { signal?: AbortSignal; done: () => void } {
+  if (typeof AbortController === 'undefined') return { signal, done: () => undefined };
+  if (!signal && ms === undefined) return { done: () => undefined };
+  const controller = new AbortController();
+  const abort = () => controller.abort();
+  const timer = ms !== undefined ? setTimeout(abort, ms) : undefined;
+  if (signal?.aborted) controller.abort();
+  else signal?.addEventListener('abort', abort, { once: true });
+  return {
+    signal: controller.signal,
+    done: () => {
+      if (timer !== undefined) clearTimeout(timer);
+      signal?.removeEventListener('abort', abort);
+    },
+  };
+}

src/lib/totp-login.ts

@@ -29,6 +29,7 @@ import { sha256Hex } from './sha256';
 import { base64UrlEncode, randomBytes, randomHex } from './random';
 import { HANDOFF_REDIRECT_URI, type OAuthTokens } from './oauth';
 import { NetworkError } from '../api/jmap-client';
+import { signalWithTimeout, throwIfCancelled } from './sign-in-abort';
 
 // Stalwart 0.16's built-in OAuth accepts any client id; the webmail's public
 // one keeps refresh behaviour identical to the browser-handoff bundle.
@@ -134,13 +135,6 @@ function rememberClient(base: string, client: LoginClient): void {
   loginClients.set(base, client);
 }
 
-function withTimeout(ms: number): { signal?: AbortSignal; done: () => void } {
-  if (typeof AbortController === 'undefined') return { done: () => undefined };
-  const controller = new AbortController();
-  const timer = setTimeout(() => controller.abort(), ms);
-  return { signal: controller.signal, done: () => clearTimeout(timer) };
-}
-
 // The password and the PKCE verifier must not follow a redirect to another
 // host. The requests go out with fetchWithoutForeignRedirect, which hands such
 // a redirect back unfollowed; where the platform followed it anyway, the
@@ -163,8 +157,12 @@ function assertSameServer(requested: string, response: Response): void {
  * names, which may be on the public address it advertises rather than the one
  * the user typed; the id is sealed with the server's key, so it serves both.
  */
-async function registerPublicClient(base: string, url = `${base}/auth/register`): Promise<LoginClient> {
-  const t = withTimeout(CLIENT_CHECK_TIMEOUT_MS);
+async function registerPublicClient(
+  base: string,
+  url = `${base}/auth/register`,
+  signal?: AbortSignal,
+): Promise<LoginClient> {
+  const t = signalWithTimeout(signal, CLIENT_CHECK_TIMEOUT_MS);
   let response: Response;
   try {
     response = await fetchWithoutForeignRedirect(url, {
@@ -182,6 +180,7 @@ async function registerPublicClient(base: string, url = `${base}/auth/register`)
       timeoutMs: CLIENT_CHECK_TIMEOUT_MS,
     });
   } catch (err) {
+    throwIfCancelled(signal);
     throw new NetworkError(err instanceof Error ? err.message : 'Server unreachable');
   } finally {
     t.done();
@@ -207,6 +206,7 @@ async function checkClient(
   base: string,
   clientId: string,
   redirectUri: string,
+  signal?: AbortSignal,
 ): Promise<'known' | 'unknown' | 'none' | 'busy'> {
   const query = new URLSearchParams({
     response_type: 'code',
@@ -217,7 +217,7 @@ async function checkClient(
     code_challenge_method: 'S256',
   });
   const url = `${base}/api/auth/client?${query.toString()}`;
-  const t = withTimeout(CLIENT_CHECK_TIMEOUT_MS);
+  const t = signalWithTimeout(signal, CLIENT_CHECK_TIMEOUT_MS);
   let response: Response;
   try {
     response = await fetchWithoutForeignRedirect(url, {
@@ -226,6 +226,7 @@ async function checkClient(
       timeoutMs: CLIENT_CHECK_TIMEOUT_MS,
     });
   } catch (err) {
+    throwIfCancelled(signal);
     throw new NetworkError(err instanceof Error ? err.message : 'Server unreachable');
   } finally {
     t.done();
@@ -253,14 +254,15 @@ async function checkClient(
  */
 export async function resolveLoginClient(
   serverUrl: string,
-  opts?: { registrationEndpoint?: string },
+  opts?: { registrationEndpoint?: string; signal?: AbortSignal },
 ): Promise<LoginClient> {
   const base = trimBase(serverUrl);
   const cached = loginClients.get(base);
   if (cached) return cached;
   const fallback: LoginClient = { clientId: DEFAULT_CLIENT_ID, redirectUri: HANDOFF_REDIRECT_URI };
+  const signal = opts?.signal;
 
-  const own = await checkClient(base, MOBILE_CLIENT_ID, REGISTERED_REDIRECT_URI);
+  const own = await checkClient(base, MOBILE_CLIENT_ID, REGISTERED_REDIRECT_URI, signal);
   if (own === 'busy') return fallback;
   if (own === 'known') {
     const client = { clientId: MOBILE_CLIENT_ID, redirectUri: REGISTERED_REDIRECT_URI };
@@ -268,11 +270,11 @@ export async function resolveLoginClient(
     return client;
   }
   if (own === 'unknown') {
-    const shared = await checkClient(base, DEFAULT_CLIENT_ID, HANDOFF_REDIRECT_URI);
+    const shared = await checkClient(base, DEFAULT_CLIENT_ID, HANDOFF_REDIRECT_URI, signal);
     if (shared === 'busy') return fallback;
     if (shared === 'unknown') {
       const endpoint = opts?.registrationEndpoint;
-      return registerPublicClient(base, endpoint && /^https?:\/\//i.test(endpoint) ? endpoint : undefined);
+      return registerPublicClient(base, endpoint && /^https?:\/\//i.test(endpoint) ? endpoint : undefined, signal);
     }
   }
   rememberClient(base, fallback);
@@ -386,6 +388,7 @@ async function requestLoginCode(
   totp: string | undefined,
   client: LoginClient,
   challenge: string,
+  signal?: AbortSignal,
 ): Promise<StalwartLoginOutcome> {
   const state = randomHex(16);
   const url = `${base}/api/auth`;
@@ -403,8 +406,10 @@ async function requestLoginCode(
         codeChallenge: challenge,
         state,
       })),
+      signal,
     });
   } catch (err) {
+    throwIfCancelled(signal);
     throw new NetworkError(err instanceof Error ? err.message : 'Server unreachable');
   }
   assertSameServer(url, loginResponse);
@@ -434,26 +439,28 @@ export async function exchangePasswordForTokens(
   username: string,
   password: string,
   totp?: string,
-  opts?: { clientId?: string; redirectUri?: string },
+  opts?: { clientId?: string; redirectUri?: string; signal?: AbortSignal },
 ): Promise<OAuthTokens> {
   const base = trimBase(serverUrl);
   const verifier = generateCodeVerifier();
   const challenge = generateCodeChallenge(verifier);
+  const signal = opts?.signal;
+  throwIfCancelled(signal);
 
   let client: LoginClient = opts?.clientId
     ? { clientId: opts.clientId, redirectUri: opts.redirectUri ?? HANDOFF_REDIRECT_URI }
-    : await resolveLoginClient(base);
-  let outcome = await requestLoginCode(base, username, password, totp, client, challenge);
+    : await resolveLoginClient(base, { signal });
+  let outcome = await requestLoginCode(base, username, password, totp, client, challenge, signal);
   // A remembered client the server no longer knows (another server now
   // answers at the address, or its key changed), or the default taken while
   // the client check failed: ask again, once. 1.0 checks the client before
   // the credentials, so this costs no failed sign-in.
   if (outcome.kind === 'rejected' && outcome.error === 'invalid_client' && !opts?.clientId) {
     loginClients.delete(base);
-    const fresh = await resolveLoginClient(base);
+    const fresh = await resolveLoginClient(base, { signal });
     if (fresh.clientId !== client.clientId) {
       client = fresh;
-      outcome = await requestLoginCode(base, username, password, totp, client, challenge);
+      outcome = await requestLoginCode(base, username, password, totp, client, challenge, signal);
     }
   }
 
@@ -486,8 +493,10 @@ export async function exchangePasswordForTokens(
       method: 'POST',
       headers: { 'Content-Type': 'application/x-www-form-urlencoded', Accept: 'application/json' },
       body: tokenParams.toString(),
+      signal,
     });
   } catch (err) {
+    throwIfCancelled(signal);
     throw new NetworkError(err instanceof Error ? err.message : 'Server unreachable');
   }
   assertSameServer(tokenUrl, tokenResponse);

src/screens/LoginScreen.tsx

@@ -33,6 +33,8 @@ import ServerStep from './login/ServerStep';
 import ConfirmStep from './login/ConfirmStep';
 import PasswordStep from './login/PasswordStep';
 import SigningInStep, { type SigningInPhase } from './login/SigningInStep';
+import { loginBackAction } from './login/login-back';
+import { isSignInCancelled } from '../lib/sign-in-abort';
 
 interface LoginScreenProps {
   onLogin?: () => void;
@@ -117,6 +119,33 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
   // (in add mode the server fields still name the signed-in account's host).
   const [pairingWebmailUrl, setPairingWebmailUrl] = React.useState<string | null>(null);
 
+  // One controller per sign-in or server search (U-133). Cancel and Back
+  // abort it; a flow whose controller is no longer the current one drops
+  // whatever its awaits bring back, so a late answer neither signs in nor
+  // shows an error over the form the user went back to.
+  const attemptRef = React.useRef<AbortController | null>(null);
+  const beginAttempt = React.useCallback((): AbortController => {
+    attemptRef.current?.abort();
+    const controller = new AbortController();
+    attemptRef.current = controller;
+    return controller;
+  }, []);
+  const isLive = (attempt: AbortController) => attemptRef.current === attempt && !attempt.signal.aborted;
+  const endAttempt = React.useCallback((attempt: AbortController): boolean => {
+    if (attemptRef.current !== attempt) return false;
+    attemptRef.current = null;
+    return true;
+  }, []);
+  const cancelSignIn = React.useCallback(() => {
+    attemptRef.current?.abort();
+    attemptRef.current = null;
+    setBusy(null);
+    setSearching(false);
+    setPairingWebmailUrl(null);
+    setNotice(null);
+  }, []);
+  React.useEffect(() => () => attemptRef.current?.abort(), []);
+
   // Addresses typed before on this device (last 5) plus the registry's, so a
   // returning user taps instead of retyping. Never contains passwords.
   const [recentEmails, setRecentEmails] = React.useState<string[]>([]);
@@ -216,27 +245,36 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
   // out of the flow entirely from the first step.
   React.useEffect(() => {
     const subscription = BackHandler.addEventListener('hardwareBackPress', () => {
-      if (busy || searching) return true;
-      if (scannerVisible) {
-        setScannerVisible(false);
-        return true;
+      const action = loginBackAction({
+        busy: !!busy,
+        searching,
+        scannerVisible,
+        pasteVisible,
+        historyLength: history.length,
+        canLeave: isAddMode && !!onCancel,
+      });
+      switch (action) {
+        case 'cancel':
+          cancelSignIn();
+          return true;
+        case 'close-scanner':
+          setScannerVisible(false);
+          return true;
+        case 'close-paste':
+          setPasteVisible(false);
+          return true;
+        case 'step-back':
+          goBack();
+          return true;
+        case 'leave':
+          onCancel?.();
+          return true;
+        default:
+          return false;
       }
-      if (pasteVisible) {
-        setPasteVisible(false);
-        return true;
-      }
-      if (history.length > 0) {
-        goBack();
-        return true;
-      }
-      if (isAddMode && onCancel) {
-        onCancel();
-        return true;
-      }
-      return false;
     });
     return () => subscription.remove();
-  }, [busy, searching, scannerVisible, pasteVisible, history.length, goBack, isAddMode, onCancel]);
+  }, [busy, searching, scannerVisible, pasteVisible, history.length, goBack, isAddMode, onCancel, cancelSignIn]);
 
   // ── flows ──────────────────────────────────────────────
 
@@ -279,21 +317,24 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
   const runHandoff = React.useCallback(
     async (target: string) => {
       setNotice(null);
+      const attempt = beginAttempt();
       setBusy('browser');
       const wasAuthenticated = useAuthStore.getState().isAuthenticated;
       try {
-        await loginViaWebmail(target, { addAccount: isAddMode });
+        await loginViaWebmail(target, { addAccount: isAddMode, signal: attempt.signal });
+        if (!isLive(attempt)) return;
         // loginViaWebmail resolves quietly when the user closes the browser —
         // that's a cancellation, not a failure, so leave them where they were.
         finishIfSignedIn(wasAuthenticated);
       } catch (err) {
+        if (!isLive(attempt) || isSignInCancelled(err)) return;
         if (continueWithSecondFactor(err, target)) return;
         setNotice(describeLoginError(err, { serverUrl: target, t }));
       } finally {
-        setBusy(null);
+        if (endAttempt(attempt)) setBusy(null);
       }
     },
-    [continueWithSecondFactor, finishIfSignedIn, isAddMode, loginViaWebmail, t],
+    [beginAttempt, continueWithSecondFactor, endAttempt, finishIfSignedIn, isAddMode, loginViaWebmail, t],
   );
 
   const handleEmailContinue = React.useCallback(async () => {
@@ -307,10 +348,12 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
     }
 
     setNotice(null);
+    const attempt = beginAttempt();
     setSearching(true);
     rememberEmail(value);
     try {
-      const found = await discoverServerForEmail(value, { knownServerUrls });
+      const found = await discoverServerForEmail(value, { knownServerUrls, signal: attempt.signal });
+      if (!isLive(attempt)) return;
       if (found) {
         setServerUrl(found);
         setServerInput(found);
@@ -322,9 +365,9 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
         goTo('server');
       }
     } finally {
-      setSearching(false);
+      if (endAttempt(attempt)) setSearching(false);
     }
-  }, [email, goTo, knownServerUrls, rememberEmail, t]);
+  }, [beginAttempt, email, endAttempt, goTo, knownServerUrls, rememberEmail, t]);
 
   const handleServerContinue = React.useCallback(() => {
     const normalized = normalizeServerUrl(serverInput);
@@ -353,15 +396,18 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
         return;
       }
       setNotice(null);
+      const attempt = beginAttempt();
       setBusy('connecting');
       const wasAuthenticated = useAuthStore.getState().isAuthenticated;
       try {
-        await loginWithToken(target, accessToken, { addAccount: isAddMode });
+        await loginWithToken(target, accessToken, { addAccount: isAddMode, signal: attempt.signal });
+        if (!isLive(attempt)) return;
         finishIfSignedIn(wasAuthenticated);
       } catch (err) {
+        if (!isLive(attempt) || isSignInCancelled(err)) return;
         setNotice(describeLoginError(err, { serverUrl: target, t }));
       } finally {
-        setBusy(null);
+        if (endAttempt(attempt)) setBusy(null);
       }
       return;
     }
@@ -377,15 +423,19 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
     }
 
     setNotice(null);
+    const attempt = beginAttempt();
     setBusy('connecting');
     const wasAuthenticated = useAuthStore.getState().isAuthenticated;
     try {
       await login(target, email.trim(), password, {
         addAccount: isAddMode,
         totp: totpRequired ? totp.trim() : undefined,
+        signal: attempt.signal,
       });
+      if (!isLive(attempt)) return;
       finishIfSignedIn(wasAuthenticated);
     } catch (err) {
+      if (!isLive(attempt) || isSignInCancelled(err)) return;
       if (err instanceof Error && err.name === 'TotpRequiredError') {
         setTotpRequired(true);
         setNotice(describeLoginError(err, { serverUrl: target, t }));
@@ -393,9 +443,9 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
       }
       setNotice(describeLoginError(err, { serverUrl: target, t }));
     } finally {
-      setBusy(null);
+      if (endAttempt(attempt)) setBusy(null);
     }
-  }, [accessToken, email, finishIfSignedIn, isAddMode, login, loginWithToken, password, serverInput, serverUrl, tokenMode, totp, totpRequired, t]);
+  }, [accessToken, beginAttempt, email, endAttempt, finishIfSignedIn, isAddMode, login, loginWithToken, password, serverInput, serverUrl, tokenMode, totp, totpRequired, t]);
 
   // A sign-in code, however it arrived: scanned, pasted, or a tapped
   // `bulwarkmail://` link.
@@ -415,23 +465,28 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
 
       // Cross-device pairing code: redeem it, no browser needed.
       setNotice(null);
+      const attempt = beginAttempt();
       setPairingWebmailUrl(payload.webmailUrl);
       setBusy('pairing');
       const wasAuthenticated = useAuthStore.getState().isAuthenticated;
       try {
-        await loginViaPairing(payload.webmailUrl, payload.code, { addAccount: isAddMode });
+        await loginViaPairing(payload.webmailUrl, payload.code, { addAccount: isAddMode, signal: attempt.signal });
+        if (!isLive(attempt)) return;
         finishIfSignedIn(wasAuthenticated);
       } catch (err) {
+        if (!isLive(attempt) || isSignInCancelled(err)) return;
         if (continueWithSecondFactor(err, payload.webmailUrl)) return;
         // The error names its own host: the webmail for a failed redeem, the
         // mail server when connecting afterwards failed.
         setNotice(describeLoginError(err, { serverUrl: payload.webmailUrl, t }));
       } finally {
-        setBusy(null);
-        setPairingWebmailUrl(null);
+        if (endAttempt(attempt)) {
+          setBusy(null);
+          setPairingWebmailUrl(null);
+        }
       }
     },
-    [continueWithSecondFactor, finishIfSignedIn, isAddMode, loginViaPairing, runHandoff, t],
+    [beginAttempt, continueWithSecondFactor, endAttempt, finishIfSignedIn, isAddMode, loginViaPairing, runHandoff, t],
   );
 
   const handleScanned = React.useCallback(
@@ -544,6 +599,7 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
         phase={busy}
         serverUrl={pairing ? pairingWebmailUrl : serverUrl || knownServerUrl}
         email={pairing || tokenMode ? null : email.trim() || null}
+        onCancel={cancelSignIn}
       />
     );
   }
@@ -565,7 +621,11 @@ export default function LoginScreen({ onLogin, isAddMode = false, onCancel }: Lo
 
   const canGoBack = history.length > 0;
   const shellProps = {
-    onBack: canGoBack ? goBack : undefined,
+    // Back during the server search stops it as well (U-133).
+    onBack: canGoBack ? () => {
+      if (searching) cancelSignIn();
+      goBack();
+    } : undefined,
     onClose: isAddMode && onCancel ? onCancel : undefined,
   };
 

src/screens/login/SigningInStep.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { View, Text, StyleSheet, ActivityIndicator, Image } from 'react-native';
+import { View, Text, StyleSheet, ActivityIndicator, Image, Pressable } from 'react-native';
 import { SafeAreaView } from 'react-native-safe-area-context';
 import { spacing, radius, typography, componentSizes, type ThemePalette } from '../../theme/tokens';
 import { useColors, useResolvedTheme } from '../../theme/colors';
@@ -16,6 +16,8 @@ interface SigningInStepProps {
   phase: SigningInPhase;
   serverUrl: string | null;
   email: string | null;
+  /** Stops the sign-in and goes back to the step it started from (U-133). */
+  onCancel?: () => void;
 }
 
 function hostOf(url: string | null): string {
@@ -32,7 +34,7 @@ function initialsOf(email: string): string {
  * current stage makes a slow first connection legible instead of a bare
  * spinner that could mean anything.
  */
-export default function SigningInStep({ phase, serverUrl, email }: SigningInStepProps) {
+export default function SigningInStep({ phase, serverUrl, email, onCancel }: SigningInStepProps) {
   const c = useColors();
   const theme = useResolvedTheme();
   const styles = React.useMemo(() => makeStyles(c), [c]);
@@ -68,6 +70,18 @@ export default function SigningInStep({ phase, serverUrl, email }: SigningInStep
         <Text style={styles.detail}>{copy[phase].detail}</Text>
 
         <ActivityIndicator size="small" color={c.primary} style={styles.spinner} />
+
+        {onCancel ? (
+          <Pressable
+            onPress={onCancel}
+            hitSlop={8}
+            style={({ pressed }) => [styles.cancel, pressed && styles.cancelPressed]}
+            accessibilityRole="button"
+            accessibilityLabel={t('login.mobile.cancel_sign_in', 'Cancel sign-in')}
+          >
+            <Text style={styles.cancelText}>{t('common.cancel', 'Cancel')}</Text>
+          </Pressable>
+        ) : null}
       </View>
     </SafeAreaView>
   );
@@ -98,5 +112,15 @@ function makeStyles(c: ThemePalette) {
     title: { ...typography.h2, color: c.text, textAlign: 'center' },
     detail: { ...typography.body, color: c.textSecondary, textAlign: 'center' },
     spinner: { marginTop: spacing.xl },
+    cancel: {
+      marginTop: spacing.lg,
+      minHeight: componentSizes.buttonLg,
+      paddingHorizontal: spacing.lg,
+      alignItems: 'center',
+      justifyContent: 'center',
+      borderRadius: radius.md,
+    },
+    cancelPressed: { opacity: 0.6 },
+    cancelText: { ...typography.bodyMedium, color: c.primary },
   });
 }

src/screens/login/__tests__/login-back.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { loginBackAction, type LoginBackState } from '../login-back';
+
+const idle: LoginBackState = {
+  busy: false,
+  searching: false,
+  scannerVisible: false,
+  pasteVisible: false,
+  historyLength: 2,
+  canLeave: true,
+};
+
+describe('Back on the sign-in screen', () => {
+  it('cancels a sign-in under way instead of swallowing the key (U-133)', () => {
+    expect(loginBackAction({ ...idle, busy: true })).toBe('cancel');
+  });
+
+  it('cancels the server search', () => {
+    expect(loginBackAction({ ...idle, searching: true })).toBe('cancel');
+  });
+
+  it('closes the scanner, then the paste dialog, then goes a step back', () => {
+    expect(loginBackAction({ ...idle, scannerVisible: true })).toBe('close-scanner');
+    expect(loginBackAction({ ...idle, pasteVisible: true })).toBe('close-paste');
+    expect(loginBackAction(idle)).toBe('step-back');
+  });
+
+  it('leaves add-account from the first step, and lets the system have the key otherwise', () => {
+    expect(loginBackAction({ ...idle, historyLength: 0 })).toBe('leave');
+    expect(loginBackAction({ ...idle, historyLength: 0, canLeave: false })).toBe('exit');
+  });
+});

src/screens/login/login-back.ts

@@ -0,0 +1,26 @@
+// What Android Back (and the on-screen back) does on the sign-in screen.
+// While a sign-in or the server search runs, Back cancels it and returns to
+// the step it started from (U-133); it used to be swallowed, which left a
+// hung "Signing in" with no way out but killing the app.
+
+export type LoginBackAction = 'cancel' | 'close-scanner' | 'close-paste' | 'step-back' | 'leave' | 'exit';
+
+export interface LoginBackState {
+  busy: boolean;
+  searching: boolean;
+  scannerVisible: boolean;
+  pasteVisible: boolean;
+  /** Steps behind the current one. */
+  historyLength: number;
+  /** Add-account mode with a way to close the screen. */
+  canLeave: boolean;
+}
+
+export function loginBackAction(state: LoginBackState): LoginBackAction {
+  if (state.busy || state.searching) return 'cancel';
+  if (state.scannerVisible) return 'close-scanner';
+  if (state.pasteVisible) return 'close-paste';
+  if (state.historyLength > 0) return 'step-back';
+  if (state.canLeave) return 'leave';
+  return 'exit';
+}

src/stores/__tests__/auth-store-cancel.test.ts

@@ -0,0 +1,101 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+// U-133: a sign-in that hangs had no way out. The sign-in screen now hands
+// the store an AbortSignal; once it fires, the sign-in rejects at once, the
+// client goes back to what it served before, and nothing of the account the
+// sign-in was adding stays behind, even when the connect finishes later.
+
+let finishConnect: ((value: unknown) => void) | null = null;
+
+vi.mock('../../api/jmap-client', () => ({
+  jmapClient: {
+    connect: vi.fn(() => new Promise((resolve) => { finishConnect = resolve; })),
+    snapshot: vi.fn(() => ({ session: 'previous', credentials: null, accountId: 'prev' })),
+    restoreSnapshot: vi.fn(),
+    reset: vi.fn(),
+    clearAccountCredentials: vi.fn(async () => undefined),
+    onAuthFailure: vi.fn(() => () => undefined),
+    onTokenRefresh: vi.fn(() => () => undefined),
+  },
+  AuthenticationError: class AuthenticationError extends Error {},
+  InvalidTokenError: class InvalidTokenError extends Error {},
+  NetworkError: class NetworkError extends Error {},
+  SessionSupersededError: class SessionSupersededError extends Error {},
+}));
+
+vi.mock('../../lib/push-notifications', () => ({
+  teardownPushNotifications: vi.fn(async () => undefined),
+  teardownPushNotificationsForAccount: vi.fn(async () => undefined),
+}));
+
+import { jmapClient } from '../../api/jmap-client';
+import { useAuthStore } from '../auth-store';
+import { useAccountStore } from '../account-store';
+
+const SERVER = 'https://mail.example.com';
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  finishConnect = null;
+  useAccountStore.setState({ accounts: [], activeAccountId: null });
+  useAuthStore.setState({ isAuthenticated: false, isLoading: false, error: null });
+});
+
+async function connectStarted(): Promise<void> {
+  await vi.waitFor(() => expect(finishConnect).not.toBeNull());
+}
+
+describe('cancelling a sign-in', () => {
+  it('rejects at once while the server has not answered, and keeps no error', async () => {
+    const controller = new AbortController();
+    const pending = useAuthStore.getState().login(SERVER, 'ada@example.com', 'pw', { signal: controller.signal });
+    await connectStarted();
+
+    controller.abort();
+    await expect(pending).rejects.toMatchObject({ name: 'SignInCancelledError' });
+
+    expect(useAuthStore.getState()).toMatchObject({ isLoading: false, error: null, isAuthenticated: false });
+    // The client must not go on serving the half-connected account.
+    expect(jmapClient.reset).toHaveBeenCalled();
+    expect(useAccountStore.getState().accounts).toEqual([]);
+  });
+
+  it('drops the credentials a connect stores after the user cancelled', async () => {
+    const controller = new AbortController();
+    const pending = useAuthStore.getState().login(SERVER, 'ada@example.com', 'pw', { signal: controller.signal });
+    await connectStarted();
+    controller.abort();
+    await expect(pending).rejects.toMatchObject({ name: 'SignInCancelledError' });
+
+    finishConnect!({ apiUrl: `${SERVER}/jmap/` });
+    await vi.waitFor(() => {
+      expect(jmapClient.clearAccountCredentials).toHaveBeenCalledWith('ada@example.com@mail.example.com');
+    });
+    expect(useAccountStore.getState().accounts).toEqual([]);
+    expect(useAuthStore.getState().isAuthenticated).toBe(false);
+  });
+
+  it('puts the account in use back when adding one is cancelled', async () => {
+    useAuthStore.setState({ isAuthenticated: true });
+    const controller = new AbortController();
+    const pending = useAuthStore.getState().login(SERVER, 'bob@example.com', 'pw', {
+      addAccount: true,
+      signal: controller.signal,
+    });
+    await connectStarted();
+    controller.abort();
+    await expect(pending).rejects.toMatchObject({ name: 'SignInCancelledError' });
+
+    expect(jmapClient.restoreSnapshot).toHaveBeenCalledWith({ session: 'previous', credentials: null, accountId: 'prev' });
+    expect(useAuthStore.getState().isAuthenticated).toBe(true);
+  });
+
+  it('does not start when the signal fired before', async () => {
+    const controller = new AbortController();
+    controller.abort();
+    await expect(
+      useAuthStore.getState().loginWithToken(SERVER, 'tok', { signal: controller.signal }),
+    ).rejects.toMatchObject({ name: 'SignInCancelledError' });
+    expect(useAuthStore.getState().error).toBeNull();
+  });
+});

src/stores/__tests__/auth-store-pairing.test.ts

@@ -132,7 +132,7 @@ describe('loginViaPairing with an app password', () => {
 
     await useAuthStore.getState().loginViaPairing(WEBMAIL, newCode());
 
-    expect(connect).toHaveBeenCalledWith(SERVER, 'ada@example.com', 'app-pass-123', undefined);
+    expect(connect).toHaveBeenCalledWith(SERVER, 'ada@example.com', 'app-pass-123', undefined, undefined);
     expect(connectWithOAuth).not.toHaveBeenCalled();
     expect(useAuthStore.getState()).toMatchObject({ isAuthenticated: true, username: 'ada@example.com', serverUrl: SERVER });
   });

src/stores/auth-store.ts

@@ -41,6 +41,7 @@ import {
 import { reconcileServerFingerprint } from '../lib/server-change';
 import { deviceSyncSignedIn, releaseDeviceSyncBeforeSignOut } from '../device-sync/app/lifecycle';
 import { perfMark } from '../lib/perf-marks';
+import { abortable, isSignInCancelled, throwIfCancelled } from '../lib/sign-in-abort';
 
 // Persist middleware hydrates asynchronously on cold start. Without this
 // guard, restoreSession() can read the account-store before AsyncStorage has
@@ -97,14 +98,14 @@ export interface AuthState {
     serverUrl: string,
     username: string,
     password: string,
-    opts?: { addAccount?: boolean; totp?: string },
+    opts?: SignInOptions & { totp?: string },
   ) => Promise<void>;
   /** Access-token (Bearer) sign-in with a token the user pasted, e.g. a Fastmail API token. */
-  loginWithToken: (serverUrl: string, token: string, opts?: { addAccount?: boolean }) => Promise<void>;
-  loginViaWebmail: (webmailUrl: string, opts?: { addAccount?: boolean }) => Promise<void>;
+  loginWithToken: (serverUrl: string, token: string, opts?: SignInOptions) => Promise<void>;
+  loginViaWebmail: (webmailUrl: string, opts?: SignInOptions) => Promise<void>;
   /** OAuth/OIDC (PKCE) straight against the mail server's authorization server. */
-  loginViaOAuth: (serverUrl: string, opts?: { addAccount?: boolean }) => Promise<void>;
-  loginViaPairing: (webmailUrl: string, code: string, opts?: { addAccount?: boolean }) => Promise<void>;
+  loginViaOAuth: (serverUrl: string, opts?: SignInOptions) => Promise<void>;
+  loginViaPairing: (webmailUrl: string, code: string, opts?: SignInOptions) => Promise<void>;
   logout: () => Promise<void>;
   logoutAll: () => Promise<void>;
   switchAccount: (accountId: string) => Promise<void>;
@@ -259,6 +260,16 @@ async function switchesSettled(): Promise<void> {
   while (pendingSwitch) await pendingSwitch.settled;
 }
 
+/**
+ * `signal` is the sign-in screen's Cancel (U-133): once it fires, the sign-in
+ * rejects with SignInCancelledError, puts the connection in use back and
+ * keeps nothing of the account it was adding.
+ */
+export interface SignInOptions {
+  addAccount?: boolean;
+  signal?: AbortSignal;
+}
+
 interface SignedIn {
   session: JMAPSession;
   serverUrl: string;
@@ -276,12 +287,14 @@ interface SignedIn {
 async function signIn(
   set: (partial: Partial<AuthState>) => void,
   get: () => AuthState,
-  opts: { addAccount?: boolean } | undefined,
+  opts: SignInOptions | undefined,
   connect: () => Promise<SignedIn>,
 ): Promise<SignedIn> {
   signInsUnderWay += 1;
   let saved: typeof signInSaved = null;
+  const signal = opts?.signal;
   try {
+    throwIfCancelled(signal);
     // A switch under way lands first, so there is one account to put back.
     await switchesSettled();
     const isCurrent = beginAccountChange();
@@ -304,7 +317,23 @@ async function signIn(
     let wasRegistered = true;
     const accountStore = useAccountStore.getState();
     try {
-      signedIn = await connect();
+      const connecting = connect();
+      if (signal) {
+        // Cancelled while connecting: the connect goes on in the background
+        // until its next session check (putting the connection back below
+        // supersedes it). Should it get as far as storing credentials for
+        // the account anyway, they go too.
+        connecting.then(
+          (late) => {
+            if (!signal.aborted || signedIn) return;
+            if (!useAccountStore.getState().getAccountById(late.accountId)) {
+              void jmapClient.clearAccountCredentials(late.accountId).catch(() => undefined);
+            }
+          },
+          () => undefined,
+        );
+      }
+      signedIn = await abortable(connecting, signal);
       if (!isCurrent()) throw new SessionSupersededError();
       wasRegistered = !!accountStore.getAccountById(signedIn.accountId);
       // Contacts/calendar are still single-bucket, so wipe those now that
@@ -315,6 +344,7 @@ async function signIn(
       }
       await checkServerChange(signedIn.accountId, signedIn.session);
       if (!isCurrent()) throw new SessionSupersededError();
+      throwIfCancelled(signal);
       accountStore.addAccount({
         serverUrl: signedIn.serverUrl,
         username: signedIn.username,
@@ -325,7 +355,8 @@ async function signIn(
         hasError: false,
       });
     } catch (err) {
-      putBack(!!signedIn);
+      // A cancelled connect may already hold the client: take it back.
+      putBack(!!signedIn || isSignInCancelled(err));
       // connect() stored credentials for an account the registry never took
       // (the account limit, or a newer account change): none stay behind.
       if (signedIn && !wasRegistered && !useAccountStore.getState().getAccountById(signedIn.accountId)) {
@@ -394,7 +425,7 @@ function assertRoomForAccount(accountId?: string): void {
 
 // Adding an account to a full registry: refuse before a sign-in code is
 // spent or a browser sign-in started, since what they buy can't be kept.
-function refuseAddWhenFull(set: (partial: Partial<AuthState>) => void, opts?: { addAccount?: boolean }): void {
+function refuseAddWhenFull(set: (partial: Partial<AuthState>) => void, opts?: SignInOptions): void {
   if (!opts?.addAccount) return;
   try {
     assertRoomForAccount();
@@ -463,7 +494,7 @@ async function completeOAuthHandoff(
   set: (partial: Partial<AuthState>) => void,
   get: () => AuthState,
   result: Extract<HandoffResult, { flow: 'oauth' }>,
-  opts?: { addAccount?: boolean },
+  opts?: SignInOptions,
 ): Promise<void> {
   await signIn(set, get, opts, async () => {
     const { session, username, accountId } = await jmapClient.connectWithOAuth(result.serverUrl, result.tokens);
@@ -471,6 +502,23 @@ async function completeOAuthHandoff(
   });
 }
 
+// After an await of a sign-in step that takes no signal: stop when the user
+// cancelled meanwhile.
+function cancelledSignIn(set: (partial: Partial<AuthState>) => void, opts?: SignInOptions): void {
+  if (!opts?.signal?.aborted) return;
+  set({ isLoading: false, error: null });
+  throwIfCancelled(opts.signal);
+}
+
+function oauthSignInMessage(err: unknown): string | null {
+  if (isSignInCancelled(err)) return null;
+  return err instanceof AuthenticationError
+    ? 'Authentication rejected by server'
+    : err instanceof Error
+      ? err.message
+      : 'OAuth sign-in failed';
+}
+
 function applyConnectedState(
   set: (partial: Partial<AuthState>) => void,
   session: JMAPSession,
@@ -514,12 +562,16 @@ export const useAuthStore = create<AuthState>((set, get) => ({
       // Adding an additional account keeps the live connection until the
       // new sign-in succeeded, so a typo doesn't kill the current session.
       await signIn(set, get, opts, async () => {
-        const session = await jmapClient.connect(serverUrl, username, password, opts?.totp);
+        const session = await jmapClient.connect(serverUrl, username, password, opts?.totp, opts?.signal);
         const base = serverUrl.replace(/\/+$/, '');
         return { session, serverUrl: base, username, accountId: generateAccountId(username, base) };
       });
       set({ pendingTotpLogin: null });
     } catch (err) {
+      if (isSignInCancelled(err)) {
+        set({ isLoading: false, error: null });
+        throw err;
+      }
       if (err instanceof Error && err.name === 'TotpRequiredError') {
         // Keep what the user (or the webmail hand-off) supplied so the code
         // step doesn't make them retype the password.
@@ -550,7 +602,7 @@ export const useAuthStore = create<AuthState>((set, get) => ({
       });
       set({ pendingTotpLogin: null });
     } catch (err) {
-      set({ isLoading: false, error: err instanceof Error ? err.message : 'Connection failed' });
+      set({ isLoading: false, error: isSignInCancelled(err) ? null : err instanceof Error ? err.message : 'Connection failed' });
       throw err;
     }
   },
@@ -562,8 +614,11 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     // served there. Opening `/login?mobile_redirect_uri=…` on a bare Stalwart
     // lands on a 404 or the admin page, so check first and fall back to the
     // server's own OAuth (PKCE) when the webmail is missing.
-    if (!(await probeWebmail(webmailUrl))) {
+    const hasWebmail = await probeWebmail(webmailUrl);
+    cancelledSignIn(set, opts);
+    if (!hasWebmail) {
       const metadata = await discoverOAuthMetadata(webmailUrl);
+      cancelledSignIn(set, opts);
       if (metadata) {
         await get().loginViaOAuth(webmailUrl, opts);
         return;
@@ -575,12 +630,14 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     let result;
     try {
       result = await runWebmailHandoff(webmailUrl, { addAccount: opts?.addAccount });
+      cancelledSignIn(set, opts);
     } catch (err) {
       if (err instanceof HandoffCancelledError) {
         // User closed the browser tab — quiet exit, no error banner.
         set({ isLoading: false, error: null });
         return;
       }
+      if (isSignInCancelled(err)) throw err;
       const message = err instanceof Error ? err.message : 'Sign-in failed';
       set({ isLoading: false, error: message });
       throw err;
@@ -600,13 +657,7 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     try {
       await completeOAuthHandoff(set, get, result, opts);
     } catch (err) {
-      const message =
-        err instanceof AuthenticationError
-          ? 'Authentication rejected by server'
-          : err instanceof Error
-            ? err.message
-            : 'OAuth sign-in failed';
-      set({ isLoading: false, error: message });
+      set({ isLoading: false, error: oauthSignInMessage(err) });
       throw err;
     }
   },
@@ -620,11 +671,13 @@ export const useAuthStore = create<AuthState>((set, get) => ({
       const metadata = await discoverOAuthMetadata(base);
       if (!metadata) throw new HandoffError('This server does not offer OAuth sign-in');
       tokens = await loginWithPkce(base, metadata, { addAccount: opts?.addAccount });
+      cancelledSignIn(set, opts);
     } catch (err) {
       if (err instanceof HandoffCancelledError) {
         set({ isLoading: false, error: null });
         return;
       }
+      if (isSignInCancelled(err)) throw err;
       const message = err instanceof Error ? err.message : 'Sign-in failed';
       set({ isLoading: false, error: message });
       throw err;
@@ -632,13 +685,7 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     try {
       await completeOAuthHandoff(set, get, { flow: 'oauth', serverUrl: base, tokens }, opts);
     } catch (err) {
-      const message =
-        err instanceof AuthenticationError
-          ? 'Authentication rejected by server'
-          : err instanceof Error
-            ? err.message
-            : 'OAuth sign-in failed';
-      set({ isLoading: false, error: message });
+      set({ isLoading: false, error: oauthSignInMessage(err) });
       throw err;
     }
   },
@@ -660,13 +707,13 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     set({ isLoading: true, error: null });
     let result: HandoffResult;
     try {
-      result = await redeemPairingCode(webmailUrl, code);
+      result = await abortable(redeemPairingCode(webmailUrl, code), opts?.signal);
     } catch (err) {
       // Not redeemed here: the server is the judge of a retry (it answers
       // "used" if the request did get through).
       pairingCodesSeen.delete(code);
       const message = err instanceof Error ? err.message : 'Pairing failed';
-      set({ isLoading: false, error: message });
+      set({ isLoading: false, error: isSignInCancelled(err) ? null : message });
       throw err;
     }
 
@@ -677,6 +724,10 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     // keeps for that step), and the account limit, which a new code won't fix.
     const serverHost = hostOfUrl(result.serverUrl);
     const connectFailed = (err: unknown): unknown => {
+      if (isSignInCancelled(err)) {
+        set({ isLoading: false, error: null });
+        return err;
+      }
       if (err instanceof Error && (err.name === 'TotpRequiredError' || err instanceof AccountLimitError)) {
         // `login` has settled the store already; the OAuth path has not.
         if (get().isLoading) set({ isLoading: false, error: err.message });
@@ -700,7 +751,7 @@ export const useAuthStore = create<AuthState>((set, get) => ({
       // password): the normal password sign-in, which also keeps the live
       // account when adding another one fails.
       try {
-        await get().login(result.serverUrl, result.username, result.password, { addAccount: opts?.addAccount });
+        await get().login(result.serverUrl, result.username, result.password, { addAccount: opts?.addAccount, signal: opts?.signal });
       } catch (err) {
         throw connectFailed(err);
       }