Sign in on a phone with no console work, in the browser's own session

Mobile OAuth reused the desktop client all along; what stopped it was the
browser. Sending the user out to Safari or Chrome backgrounds the app, iOS
suspends it, and the redirect carrying the code arrives at a socket nobody
is accepting on. The consent page now opens in front of the app instead, in
SFSafariViewController or a Chrome Custom Tab, so the loopback listener
stays live and the existing `installed` client is enough. Verified against
Google's real consent screen on a simulator and an emulator.

A per-platform client is still supported and is now an upgrade rather than a
prerequisite. On iOS it buys ASWebAuthenticationSession, which shares
Safari's session so nobody is asked to sign in to Google twice. Android
needs nothing: Custom Tabs share Chrome's cookies, measured rather than
assumed. iOS session sharing could not be confirmed on the simulator and
wants a real device.

Never an app-owned WebView: Google blocks it, and rightly, since a webview
the app controls can read the password typed into it.

Cancelling is no longer reported as a failure. AuthEvent carries a
`cancelled` flag, set by comparing against the constant every back-out path
returns, and Google's `access_denied` on desktop counts too.

Five frontend bugs found by driving the real UI, not by reading it: the
details card slid under the tab bar leaving its buttons unhittable; the
ghost click after a touch pressed a button in the card that tap had just
opened, opening the editor by itself; the swipe that pages the day was dead
over every read-only block; 84px of macOS traffic-light lane was reserved on
platforms with no traffic lights; and the desktop header ignored the top
safe area on an iPad. A first launch now says what to do next rather than
showing an empty grid, and accounts are named as Google accounts throughout.

src-tauri/Cargo.lock

@@ -2279,10 +2279,12 @@ name = "margin-calendar"
 version = "0.1.0"
 dependencies = [
  "base64 0.22.1",
+ "block2",
  "chacha20poly1305",
  "chrono",
  "chrono-tz",
  "objc2",
+ "objc2-foundation",
  "objc2-ui-kit",
  "rand 0.8.7",
  "reqwest 0.12.28",

src-tauri/Cargo.toml

@@ -44,9 +44,10 @@ chacha20poly1305 = "0.10"
 tauri-plugin-process = "2"
 tauri-plugin-updater = "2"
 
-# One UIKit property that wry does not set for us; stop_uikit_shrinking_the_viewport in lib.rs says
-# which one and why. These versions are the ones wry already resolves for its own iOS backend, so
-# matching them keeps a single copy of objc2 in the build rather than a second incompatible one.
+# Two things wry leaves to us: one UIKit property (stop_uikit_shrinking_the_viewport in lib.rs) and
+# the SFSafariViewController that shows the consent page (google/browser.rs). These versions are the
+# ones wry already resolves for its own iOS backend, so matching them keeps a single copy of objc2 in
+# the build rather than a second incompatible one.
 [target.'cfg(target_os = "ios")'.dependencies]
 objc2 = "0.6"
 objc2-ui-kit = { version = "0.3", default-features = false, features = [
@@ -55,6 +56,14 @@ objc2-ui-kit = { version = "0.3", default-features = false, features = [
   "UIView",
   "UIScrollView",
 ] }
+objc2-foundation = { version = "0.3", default-features = false, features = [
+  "std",
+  "NSString",
+  "NSURL",
+] }
+# Only for the null completion handler the two present/dismiss calls take. Passing a raw null
+# pointer there would encode as an object rather than a block, which objc2 checks and rejects.
+block2 = "0.6"
 
 [dev-dependencies]
 tempfile = "3"

src-tauri/gen/android/app/build.gradle.kts

@@ -58,6 +58,9 @@ rust {
 }
 
 dependencies {
+    // Chrome Custom Tabs, for the OAuth consent page. Added by hand, so `tauri android init` will
+    // drop it again along with the bridge in MainActivity.kt that uses it.
+    implementation("androidx.browser:browser:1.8.0")
     implementation("androidx.webkit:webkit:1.14.0")
     implementation("androidx.appcompat:appcompat:1.7.1")
     implementation("androidx.activity:activity-ktx:1.10.1")

src-tauri/gen/android/app/src/main/AndroidManifest.xml

@@ -5,6 +5,17 @@
     <!-- AndroidTV support -->
     <uses-feature android:name="android.software.leanback" android:required="false" />
 
+    <!-- Android 11 and up hide every other package from an app unless it says which it needs to
+         see. Without this, CustomTabsClient.getPackageName finds no browser at all, the OAuth
+         consent page falls back to an ordinary browser Intent, and the loopback listener it is
+         supposed to redirect to is left in a backgrounded process. Added by hand, so
+         `tauri android init` will drop it. -->
+    <queries>
+        <intent>
+            <action android:name="android.support.customtabs.action.CustomTabsService" />
+        </intent>
+    </queries>
+
     <application
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"

src-tauri/gen/android/app/src/main/java/studio/margin/calendar/MainActivity.kt

@@ -1,18 +1,30 @@
 package studio.margin.calendar
 
+import android.content.Intent
+import android.net.Uri
 import android.os.Bundle
 import android.webkit.JavascriptInterface
 import android.webkit.WebView
 import androidx.activity.enableEdgeToEdge
+import androidx.browser.customtabs.CustomTabsClient
+import androidx.browser.customtabs.CustomTabsIntent
 import androidx.core.view.ViewCompat
 import androidx.core.view.WindowInsetsCompat
 
 /**
- * Android's WebView works out env(safe-area-inset-*) from the display cutout and from nothing else,
- * so the status and navigation bars overlap the page while env() still reads 0 underneath them. A
- * targetSdk of 36 makes edge to edge mandatory, so there is no overlap to opt out of: the only way
- * out is to measure the bars and tell the page. That is what this does, and the page turns the
- * numbers into --safe-top and --safe-bottom (src/safeArea.ts).
+ * Two things Rust cannot reach on Android without JNI, both published to the page as JavaScript
+ * bridges and both driven from Rust with webview.eval.
+ *
+ * The first is the window insets. Android's WebView works out env(safe-area-inset-*) from the
+ * display cutout and from nothing else, so the status and navigation bars overlap the page while
+ * env() still reads 0 underneath them. A targetSdk of 36 makes edge to edge mandatory, so there is
+ * no overlap to opt out of: the only way out is to measure the bars and tell the page. The page
+ * turns the numbers into --safe-top and --safe-bottom (src/safeArea.ts).
+ *
+ * The second is the Chrome Custom Tab that shows Google's consent page (src-tauri/src/google/
+ * browser.rs). It has to be a Custom Tab rather than a WebView, because Google refuses to sign
+ * anyone in through a WebView the app owns, and rather than an ordinary browser Intent, because
+ * that would put this app in the background where its loopback listener stops accepting.
  */
 class MainActivity : TauriActivity() {
   // Device pixels. Written on the UI thread by the insets listener and read on the WebView's
@@ -26,6 +38,48 @@ class MainActivity : TauriActivity() {
     @JavascriptInterface fun bottom(): Int = bottomPx
   }
 
+  inner class AuthTab {
+    /**
+     * Runs on the WebView's bridge thread rather than the UI thread, which is fine: starting an
+     * activity touches no view hierarchy. Every exit lands the user on the consent page somehow,
+     * because a sign-in that opens nothing at all is the one outcome with no way back.
+     */
+    @JavascriptInterface
+    fun open(url: String) {
+      val uri = Uri.parse(url)
+      // Null when no installed browser implements Custom Tabs, which is rare and still possible on
+      // a stripped image or an old device.
+      val browser = CustomTabsClient.getPackageName(this@MainActivity, null)
+      if (browser != null) {
+        try {
+          val tab = CustomTabsIntent.Builder().setShowTitle(true).build()
+          tab.intent.setPackage(browser)
+          tab.launchUrl(this@MainActivity, uri)
+          return
+        } catch (e: Exception) {
+          Logger.warn("could not open a custom tab: $e")
+        }
+      }
+      // The old behaviour, and a worse one: this hands the user to a separate browser app, which
+      // backgrounds this process. The listener survives a short trip but the whole point of the
+      // tab above is not to take one.
+      startActivity(Intent(Intent.ACTION_VIEW, uri))
+    }
+
+    /**
+     * A Custom Tab belongs to Chrome and cannot be closed by the app that launched it. What can be
+     * done is to bring this activity back to the front of the task the tab was launched into, which
+     * pops the tab off on the way. Same move AppAuth makes, and the background-start restrictions
+     * do not apply because this activity is already in that task's back stack.
+     */
+    @JavascriptInterface
+    fun close() {
+      val intent = Intent(this@MainActivity, MainActivity::class.java)
+      intent.flags = Intent.FLAG_ACTIVITY_CLEAR_TOP or Intent.FLAG_ACTIVITY_SINGLE_TOP
+      startActivity(intent)
+    }
+  }
+
   override fun onCreate(savedInstanceState: Bundle?) {
     enableEdgeToEdge()
     super.onCreate(savedInstanceState)
@@ -35,6 +89,7 @@ class MainActivity : TauriActivity() {
     // wry calls this between constructing the WebView and loading the URL, which is the only point
     // at which an interface can be added and still be there for the first document.
     webView.addJavascriptInterface(SafeArea(), "__androidSafeArea")
+    webView.addJavascriptInterface(AuthTab(), "__androidAuthTab")
 
     ViewCompat.setOnApplyWindowInsetsListener(webView) { view, insets ->
       // The cutout is folded in rather than trusted on its own: a landscape cutout down one side

src-tauri/src/dto.rs

@@ -174,4 +174,8 @@ pub struct AuthEvent {
     pub error: Option<String>,
     pub account_id: Option<String>,
     pub email: Option<String>,
+    /// The user closed the consent browser rather than anything going wrong. Still `ok: false`,
+    /// because no account arrived, but changing your mind is not a failure and must not be
+    /// reported as one.
+    pub cancelled: bool,
 }

src-tauri/src/google/auth.rs

@@ -7,12 +7,9 @@
 //      a different account cannot write against stale remote ids.
 
 use std::collections::HashMap;
-#[cfg(desktop)]
 use std::io::{Read, Write};
-#[cfg(desktop)]
 use std::net::{TcpListener, TcpStream};
 use std::sync::LazyLock;
-#[cfg(desktop)]
 use std::time::Instant;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
@@ -29,13 +26,20 @@ use crate::dto::{Account, AuthEvent};
 use crate::google::secrets;
 
 pub const SCOPES: &str = "openid email https://www.googleapis.com/auth/calendar";
+
+/// How long the listener waits for Google to come back. Two minutes is generous on a desktop, where
+/// the browser is a window away and a keyboard is a keyboard.
 #[cfg(desktop)]
 pub const AUTH_TIMEOUT_SECS: u64 = 120;
 
-/// How long a mobile consent round trip may take. Far longer than the desktop listener's two
-/// minutes, because on a phone the browser is a separate app: signing in, a password manager and
-/// a 2FA prompt in a third app can all happen between leaving and coming back, and this process
-/// is doing nothing but holding a verifier in the meantime.
+/// The same wait on a phone, which has to cover an email and a password typed on glass, a password
+/// manager round trip, and very often a 2FA prompt in a third app.
+#[cfg(mobile)]
+pub const AUTH_TIMEOUT_SECS: u64 = 900;
+
+/// The same again for the deep link flow, where the wait is even less this app's to control: the
+/// browser is a separate app there, so this process may be backgrounded for all of it while it does
+/// nothing but hold a verifier.
 #[cfg(mobile)]
 pub const PENDING_TIMEOUT_SECS: u64 = 900;
 
@@ -53,10 +57,14 @@ pub static HTTP: LazyLock<reqwest::Client> = LazyLock::new(|| {
         .expect("could not build the HTTP client")
 });
 
-/// Google issues a different OAuth client per platform and will not accept one in another's place.
-/// A desktop client is confidential and redirects to loopback; an Android or iOS client is public,
-/// has no secret at all, and redirects to a custom URI scheme. So the credentials file carries up
-/// to three clients and the build picks the one for the platform it is being compiled for.
+/// Up to three OAuth clients, of which a build uses exactly one.
+///
+/// `installed` is a Desktop client: confidential, so it has a secret, and allowed to redirect to
+/// loopback on any port without registering it first. `android` and `ios` are public clients with
+/// no secret at all, and they redirect to a custom URI scheme instead.
+///
+/// A phone uses `installed` unless its own block is present, which is a deliberate choice and the
+/// reason mobile sign-in needs no console work. `connect` says what the two flows cost.
 #[derive(Deserialize)]
 struct CredentialsFile {
     installed: Credentials,
@@ -81,6 +89,12 @@ pub struct Credentials {
     /// Mobile only, and only when Google's console shows something other than the default below.
     #[serde(default)]
     pub redirect_uri: Option<String>,
+    /// Set at load time rather than read from the file: true when this client came out of the
+    /// `android` or `ios` block. It is the only thing that decides which of the two mobile flows
+    /// runs, so it travels with the client that forces the choice rather than being worked out
+    /// again wherever the answer is needed.
+    #[serde(skip)]
+    pub platform_client: bool,
 }
 
 fn default_auth_uri() -> String {
@@ -109,20 +123,27 @@ fn reversed_client_id(client_id: &str) -> String {
 
 const NOT_SET_UP: &str = "Google Calendar is not set up yet. Add a real OAuth client to google-credentials.json and rebuild.";
 
+/// The one client this build signs in with. A missing platform block is not an error: it means the
+/// desktop client and the loopback flow, which is what a phone gets until somebody decides
+/// otherwise. Every caller has to agree on the answer, because a refresh token belongs to the
+/// client that obtained it.
 pub fn load_credentials() -> Result<Credentials, String> {
     let parsed: CredentialsFile = serde_json::from_str(CREDENTIALS_JSON)
         .map_err(|e| format!("invalid google-credentials.json: {e}"))?;
 
     #[cfg(target_os = "android")]
-    let creds = parsed.android.ok_or(
-        "google-credentials.json has no \"android\" client. Create an OAuth client of type Android in the Google Cloud console and add it. See docs/mobile.md.",
-    )?;
+    let (mut creds, platform_client) = match parsed.android {
+        Some(creds) => (creds, true),
+        None => (parsed.installed, false),
+    };
     #[cfg(target_os = "ios")]
-    let creds = parsed.ios.ok_or(
-        "google-credentials.json has no \"ios\" client. Create an OAuth client of type iOS in the Google Cloud console and add it. See docs/mobile.md.",
-    )?;
+    let (mut creds, platform_client) = match parsed.ios {
+        Some(creds) => (creds, true),
+        None => (parsed.installed, false),
+    };
     #[cfg(not(any(target_os = "android", target_os = "ios")))]
-    let creds = parsed.installed;
+    let (mut creds, platform_client) = (parsed.installed, false);
+    creds.platform_client = platform_client;
 
     if creds.client_id.starts_with("YOUR_CLIENT_ID")
         || creds
@@ -135,8 +156,9 @@ pub fn load_credentials() -> Result<Credentials, String> {
     Ok(creds)
 }
 
-/// Where Google sends the browser back to. Desktop binds a loopback port per attempt, so it is
-/// decided in `connect` rather than here and this is mobile only.
+/// Where Google sends the browser back to on the custom scheme flow. The loopback flow binds a port
+/// per attempt and works its own redirect out in `connect_by_loopback`, so this is only for the
+/// platforms that have been given their own client.
 #[cfg(mobile)]
 pub fn redirect_uri(creds: &Credentials) -> String {
     if let Some(explicit) = &creds.redirect_uri {
@@ -219,7 +241,6 @@ fn urlencode(s: &str) -> String {
     url::form_urlencoded::byte_serialize(s.as_bytes()).collect()
 }
 
-#[cfg(desktop)]
 fn write_http_message(stream: &mut TcpStream, message: &str) {
     let body = format!(
         "<!doctype html><html><head><meta charset=\"utf-8\"><title>Margin Calendar</title></head>\
@@ -235,7 +256,6 @@ fn write_http_message(stream: &mut TcpStream, message: &str) {
     let _ = stream.flush();
 }
 
-#[cfg(desktop)]
 #[derive(Debug, PartialEq, Eq)]
 enum Redirect {
     Code(String),
@@ -245,7 +265,6 @@ enum Redirect {
     Waiting,
 }
 
-#[cfg(desktop)]
 fn request_path(request: &str) -> &str {
     request
         .lines()
@@ -254,7 +273,6 @@ fn request_path(request: &str) -> &str {
         .unwrap_or("")
 }
 
-#[cfg(desktop)]
 fn parse_redirect(path: &str, expected_state: &str) -> Redirect {
     if path == "/favicon.ico" {
         return Redirect::Waiting;
@@ -284,7 +302,14 @@ fn parse_redirect(path: &str, expected_state: &str) -> Redirect {
     }
 }
 
-#[cfg(desktop)]
+/// What the frontend is told when the user backed out rather than finishing: closing the consent
+/// browser on a phone, or pressing Cancel on Google's own screen anywhere.
+///
+/// `emit_auth` compares against this exact string to set `AuthEvent.cancelled`, which is what stops
+/// the frontend reporting a change of mind as a failure. So it is a constant on every platform, and
+/// every path that means "the user chose not to" must return this rather than wording its own.
+const CANCELLED: &str = "Sign-in was cancelled.";
+
 fn await_code(
     listener: TcpListener,
     expected_state: &str,
@@ -295,6 +320,13 @@ fn await_code(
         if Instant::now() > deadline {
             return Err("Timed out waiting for Google authorization.".to_string());
         }
+        // Closing the consent page is the mobile equivalent of closing the browser tab, and the
+        // one abandonment the OS actually tells us about. Polled here rather than interrupting the
+        // loop, so this stays the only place that decides an attempt is over.
+        #[cfg(mobile)]
+        if crate::google::browser::cancelled() {
+            return Err(CANCELLED.to_string());
+        }
         match listener.accept() {
             Ok((mut stream, _)) => {
                 stream.set_nonblocking(false).ok();
@@ -315,6 +347,12 @@ fn await_code(
                             &mut stream,
                             "Authorization was cancelled. You can close this tab.",
                         );
+                        // `access_denied` is Google's word for the user pressing Cancel on the
+                        // consent screen, which is the same decision as closing the browser and
+                        // deserves the same quiet handling. Anything else really did go wrong.
+                        if error == "access_denied" {
+                            return Err(CANCELLED.to_string());
+                        }
                         return Err(format!("Google authorization failed: {error}"));
                     }
                     Redirect::Mismatch => {
@@ -491,9 +529,12 @@ fn auth_url(creds: &Credentials, redirect: &str, challenge: &str, csrf: &str) ->
     )
 }
 
-/// The system browser, never an in-app webview. Google blocks the embedded-webview flow outright,
-/// and it deserves to be blocked: a webview the app controls can read what the user types into it.
-fn open_in_browser(app: &tauri::AppHandle, url: &str) {
+/// Hands the URL to whichever browser the OS considers the user's, in its own process. Never an
+/// in-app webview: Google blocks the embedded-webview flow outright, and it deserves to be blocked,
+/// because a webview the app controls can read what the user types into it. `browser.rs` covers the
+/// mobile surfaces, which are the system's browser too and are only in front of the app rather than
+/// inside it.
+pub(super) fn open_in_browser(app: &tauri::AppHandle, url: &str) {
     use tauri_plugin_opener::OpenerExt;
     let _ = app.opener().open_url(url.to_string(), None::<&str>);
 }
@@ -507,10 +548,14 @@ fn emit_auth(app: &tauri::AppHandle, outcome: Result<(String, String), String>)
                 error: None,
                 account_id: Some(account_id),
                 email: Some(email),
+                cancelled: false,
             }
         }
+        // Compared against the constant the cancel paths raise, rather than matched on its text,
+        // so rewording it cannot quietly turn a cancel back into an error on screen.
         Err(error) => AuthEvent {
             ok: false,
+            cancelled: error == CANCELLED,
             error: Some(error),
             account_id: None,
             email: None,
@@ -519,9 +564,34 @@ fn emit_auth(app: &tauri::AppHandle, outcome: Result<(String, String), String>)
     let _ = app.emit("auth", event);
 }
 
-#[cfg(desktop)]
+/// One consent request, on all five platforms.
+///
+/// The loopback flow is the default everywhere, phones included. Google lets a Desktop client
+/// redirect to loopback on any port with nothing registered in advance, and the token endpoint
+/// checks the client id, the secret and the redirect and has no idea which OS asked. What used to
+/// make this impossible on a phone was not the protocol but the browser: sending the user out to
+/// Safari suspends this process, and a suspended process is not accepting on its socket. An in-app
+/// browser does not leave, which is why `browser.rs` exists and why this branch is now the common
+/// one.
+///
+/// The custom scheme flow runs instead when the credentials file has a client for this platform.
+/// That is Google's stated guidance, and the thing to reach for if they ever start enforcing it,
+/// but on iOS it is worth having for its own sake: it is the only way to reach
+/// `ASWebAuthenticationSession`, which is the only iOS browser that shares Safari's cookies.
+/// docs/mobile.md has the trade in full.
 pub async fn connect(app: tauri::AppHandle) -> Result<String, String> {
     let creds = load_credentials()?;
+    #[cfg(mobile)]
+    if creds.platform_client {
+        return connect_by_deep_link(app, creds).await;
+    }
+    connect_by_loopback(app, creds).await
+}
+
+async fn connect_by_loopback(
+    app: tauri::AppHandle,
+    creds: Credentials,
+) -> Result<String, String> {
     let verifier = random_b64(64);
     let challenge = pkce_challenge(&verifier);
     let csrf = random_b64(24);
@@ -531,7 +601,10 @@ pub async fn connect(app: tauri::AppHandle) -> Result<String, String> {
     let redirect = format!("http://127.0.0.1:{port}");
     let url = auth_url(&creds, &redirect, &challenge, &csrf);
 
+    #[cfg(desktop)]
     open_in_browser(&app, &url);
+    #[cfg(mobile)]
+    crate::google::browser::open(&app, &url);
 
     let app_bg = app.clone();
     tauri::async_runtime::spawn(async move {
@@ -542,13 +615,26 @@ pub async fn connect(app: tauri::AppHandle) -> Result<String, String> {
     Ok(url)
 }
 
-/// Mobile has no loopback listener to wait on, so `connect` ends the moment the browser opens and
-/// the flow resumes in `handle_redirect` whenever the deep link arrives. What is stashed here is
-/// the PKCE verifier and the CSRF state, which is the only thing tying the code that comes back to
-/// the request that went out.
+/// The custom scheme flow. What is stashed here is the PKCE verifier and the CSRF state, which is
+/// the only thing tying the code that comes back to the request that went out, since there is no
+/// listener holding either on its own stack.
+///
+/// Where the answer comes back from differs by platform, and so does what it is worth.
+///
+/// iOS hands the URL to `ASWebAuthenticationSession`, which reports the callback straight to a
+/// completion handler. It is the only iOS surface that shares Safari's cookies, so an account
+/// already signed in on the phone is offered by name rather than asking for a password again. That
+/// is the reason this flow is worth the console visit on iOS, and `browser.rs` says the rest.
+///
+/// Android opens the external browser and waits for the deep link, which is a genuinely separate
+/// app: this process may be backgrounded, or killed outright, for the whole of it. Android needs
+/// none of this, because a Custom Tab already shares Chrome's cookies and the loopback flow above
+/// already works, so this is only ever reached when somebody has gone and made an Android client.
 #[cfg(mobile)]
-pub async fn connect(app: tauri::AppHandle) -> Result<String, String> {
-    let creds = load_credentials()?;
+async fn connect_by_deep_link(
+    app: tauri::AppHandle,
+    creds: Credentials,
+) -> Result<String, String> {
     let verifier = random_b64(64);
     let challenge = pkce_challenge(&verifier);
     let csrf = random_b64(24);
@@ -561,15 +647,40 @@ pub async fn connect(app: tauri::AppHandle) -> Result<String, String> {
         *pending = Some(Pending {
             state: csrf,
             verifier,
-            redirect,
+            redirect: redirect.clone(),
             expires: now() + PENDING_TIMEOUT_SECS,
         });
     }
 
+    #[cfg(target_os = "ios")]
+    crate::google::browser::authenticate(&app, &url, callback_scheme(&redirect));
+    #[cfg(not(target_os = "ios"))]
     open_in_browser(&app, &url);
     Ok(url)
 }
 
+/// `ASWebAuthenticationSession` matches on the scheme alone and wants it bare, so the path and the
+/// colon that `redirect_uri` builds have to come back off.
+#[cfg(target_os = "ios")]
+fn callback_scheme(redirect: &str) -> &str {
+    redirect.split(':').next().unwrap_or(redirect)
+}
+
+/// The consent surface ended with no callback URL at all: the user closed it, or it could not be
+/// shown. Either way the pending verifier is spent, and the panel is waiting on an answer that is
+/// never coming, so it is told. A cancel says so plainly rather than reporting a failure.
+#[cfg(mobile)]
+pub async fn abandon_pending(app: tauri::AppHandle, reason: Option<String>) {
+    let auth = app.state::<AuthState>();
+    let pending = {
+        let mut slot = auth.pending.lock().await;
+        slot.take()
+    };
+    if pending.is_some() {
+        emit_auth(&app, Err(reason.unwrap_or_else(|| CANCELLED.to_string())));
+    }
+}
+
 /// Every URL the OS hands the app on a registered scheme, including ones that have nothing to do
 /// with consent. A link that carries no `state` we are waiting for is ignored in silence rather
 /// than reported, because another feature may want that scheme later and a stray link is not an
@@ -635,7 +746,6 @@ pub async fn handle_redirect(app: tauri::AppHandle, incoming: &url::Url) {
     emit_auth(&app, outcome);
 }
 
-#[cfg(desktop)]
 async fn complete_auth(
     app: &tauri::AppHandle,
     listener: TcpListener,
@@ -649,9 +759,16 @@ async fn complete_auth(
         await_code(listener, &csrf, deadline)
     })
     .await
-    .map_err(|e| e.to_string())??;
+    .map_err(|e| e.to_string())?;
 
-    finish(app, &creds, &code, &redirect, &verifier).await
+    // Nothing takes the consent page down on its own once the listener has its answer, and what it
+    // is showing by then is the listener's own "you can close this" page. Taken away here rather
+    // than after the exchange, and whatever the outcome was, so the app comes back the moment the
+    // browser has nothing left to do. Desktop has no such surface and compiles this out.
+    #[cfg(mobile)]
+    crate::google::browser::close(app);
+
+    finish(app, &creds, &code?, &redirect, &verifier).await
 }
 
 /// Code to stored account. Everything past the point where the two flows stop differing.

src-tauri/src/google/browser.rs

@@ -0,0 +1,420 @@
+// Where the consent page is shown on a phone, and how it is taken away again.
+//
+// Desktop hands the URL to the system browser and forgets about it. The loopback listener is the
+// only thing that has to survive, and a desktop process keeps running perfectly well while another
+// window has focus. A phone does not work that way: sending the user out to Safari or Chrome
+// backgrounds this process, iOS suspends it, and a suspended process accepts nothing on its socket,
+// so the redirect carrying the authorization code arrives at nobody.
+//
+// So the consent page is put in front of the app instead, by the system's own browser component.
+// Never a WebView this app owns: Google blocks that outright with `disallowed_useragent`, and it
+// deserves to be blocked, because a webview the app controls can read what is typed into it. What
+// this app gets in return for not owning it is to stay foreground, with its listener still
+// accepting, for as long as consent takes.
+//
+// Three surfaces, because the platforms do not offer the same thing.
+//
+//   Chrome Custom Tab, Android. Chrome itself, so it reads Chrome's cookie jar and an account
+//   already signed in there is offered by name. Nothing else is needed on Android.
+//
+//   SFSafariViewController, iOS, when there is no `ios` OAuth client. Safari's engine, but with
+//   storage of this app's own since iOS 11, so the user signs in from scratch inside it. It works
+//   with no console setup at all, which is the only reason to accept that.
+//
+//   ASWebAuthenticationSession, iOS, when there is one. The only iOS surface that shares Safari's
+//   session, and the one to prefer, but it intercepts a custom scheme and never an http loopback
+//   redirect, so it is reachable only where an `ios` client has bought a scheme.
+//
+// The first two are `open`/`close`: they know nothing about the answer, so the loopback listener
+// stays the thing that decides an attempt is over, and the two flags below carry the one fact it
+// cannot see for itself. `authenticate` is the third and reports its own outcome.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+
+/// Set while the consent page is up and the listener is still waiting on it. Only a dismissal
+/// during that window means anything: the same signals fire when this app takes the browser down
+/// itself, a moment after the code has already arrived.
+static WAITING: AtomicBool = AtomicBool::new(false);
+
+/// Set when the user closed the browser without finishing. `await_code` polls it rather than being
+/// interrupted, which costs up to one poll interval and keeps the listener loop the only thing that
+/// decides when a consent attempt is over.
+static DISMISSED: AtomicBool = AtomicBool::new(false);
+
+pub fn open(app: &tauri::AppHandle, url: &str) {
+    DISMISSED.store(false, Ordering::SeqCst);
+    WAITING.store(true, Ordering::SeqCst);
+    present(app, url);
+}
+
+pub fn close(app: &tauri::AppHandle) {
+    WAITING.store(false, Ordering::SeqCst);
+    dismiss(app);
+}
+
+pub fn cancelled() -> bool {
+    DISMISSED.load(Ordering::SeqCst)
+}
+
+/// The user closed the consent page: the Done button on iOS, Back or a swipe on Android.
+pub fn note_dismissed() {
+    if WAITING.load(Ordering::SeqCst) {
+        DISMISSED.store(true, Ordering::SeqCst);
+    }
+}
+
+/// iOS presents the consent page inside this app, so this process never leaves the foreground and
+/// there is nothing to hook. Android's Custom Tab is a Chrome activity on top of ours, so this app
+/// really does go to the background and coming back is the signal: the tab is in this app's own
+/// task, and the only way out of it is dismissal. `close` clears `WAITING` before it brings the
+/// activity forward, so the resume this app asks for itself is not mistaken for the user's.
+#[cfg(target_os = "android")]
+pub fn note_resumed() {
+    note_dismissed();
+}
+
+/// The other iOS surface, and the one to prefer where a custom scheme exists to make it possible.
+/// Nothing to do with the two flags above: it reports its own cancel, because it has a completion
+/// handler to report it through and no listener to interrupt.
+#[cfg(target_os = "ios")]
+pub fn authenticate(app: &tauri::AppHandle, url: &str, scheme: &str) {
+    ios::authenticate(app, url, scheme);
+}
+
+#[cfg(target_os = "ios")]
+fn present(app: &tauri::AppHandle, url: &str) {
+    ios::present(app, url);
+}
+
+#[cfg(target_os = "ios")]
+fn dismiss(app: &tauri::AppHandle) {
+    ios::dismiss(app);
+}
+
+#[cfg(target_os = "ios")]
+mod ios {
+    use block2::{DynBlock, RcBlock};
+    use objc2::rc::{Allocated, Retained};
+    use objc2::runtime::{AnyClass, AnyObject, ClassBuilder, NSObject, Sel};
+    use objc2::{msg_send, sel, ClassType};
+    use objc2_foundation::{NSError, NSString, NSURL};
+    use tauri::Manager;
+
+    use std::cell::RefCell;
+
+    // SFSafariViewController has no objc2 binding: objc2-safari-services covers the macOS extension
+    // API and nothing else. Linking the framework by hand is what puts the class in the process at
+    // all, after which it can be looked up by name.
+    #[link(name = "SafariServices", kind = "framework")]
+    extern "C" {}
+
+    thread_local! {
+        /// The presented controller and the delegate it reports to, kept only so they can be found
+        /// again: UIKit holds a delegate weakly, and a controller nothing retains is a controller
+        /// that deallocates mid-flow. Both slots are main thread only, which is where every line in
+        /// this module runs.
+        static PRESENTED: RefCell<Option<Retained<AnyObject>>> = const { RefCell::new(None) };
+        static DELEGATE: RefCell<Option<Retained<AnyObject>>> = const { RefCell::new(None) };
+    }
+
+    /// SFSafariViewController calls this when the user taps Done, and never when the dismissal came
+    /// from `dismiss` below. UIKit takes the controller off screen on its own here, so there is
+    /// nothing to do but let go of it and say what happened.
+    extern "C-unwind" fn did_finish(_this: &AnyObject, _cmd: Sel, _controller: *mut AnyObject) {
+        PRESENTED.with(|slot| slot.borrow_mut().take());
+        DELEGATE.with(|slot| slot.borrow_mut().take());
+        super::note_dismissed();
+    }
+
+    /// Registered once, lazily, because a class pair can only be registered under a given name
+    /// once per process. SFSafariViewControllerDelegate is checked with `respondsToSelector:`
+    /// rather than `conformsToProtocol:`, so declaring the one method is enough and there is no
+    /// protocol to adopt.
+    fn delegate_class() -> &'static AnyClass {
+        thread_local! {
+            static CLASS: &'static AnyClass = {
+                let mut builder = ClassBuilder::new(c"MarginConsentDelegate", NSObject::class())
+                    .expect("MarginConsentDelegate is registered once and by nobody else");
+                unsafe {
+                    builder.add_method(
+                        sel!(safariViewControllerDidFinish:),
+                        did_finish as extern "C-unwind" fn(_, _, _),
+                    );
+                }
+                builder.register()
+            };
+        }
+        CLASS.with(|class| *class)
+    }
+
+    pub fn present(app: &tauri::AppHandle, url: &str) {
+        let Some(window) = app.get_webview_window("main") else {
+            return crate::google::auth::open_in_browser(app, url);
+        };
+        let inner_app = app.clone();
+        let inner_url = url.to_string();
+        let posted = window.with_webview(move |webview| {
+            // `with_webview` hands the WKWebView over on the main thread, which is both where UIKit
+            // needs this and where the two slots above live. Its window is the app's own, so the
+            // sheet comes up over the calendar rather than over whatever else UIKit would have
+            // picked.
+            let presented = unsafe {
+                let view = webview.inner().cast::<AnyObject>();
+                let ui_window: Option<Retained<AnyObject>> = msg_send![view, window];
+                let root: Option<Retained<AnyObject>> = match &ui_window {
+                    Some(ui_window) => msg_send![&**ui_window, rootViewController],
+                    None => None,
+                };
+                match root {
+                    Some(root) => show(&root, &inner_url),
+                    None => false,
+                }
+            };
+            if !presented {
+                crate::google::auth::open_in_browser(&inner_app, &inner_url);
+            }
+        });
+        if posted.is_err() {
+            crate::google::auth::open_in_browser(app, url);
+        }
+    }
+
+    /// Everything that can be missing here is missing for the same reason: an iOS that does not
+    /// have the class, or a URL the OS will not parse. Both answer false and the caller falls back
+    /// to the external browser rather than leaving the user looking at nothing.
+    unsafe fn show(root: &AnyObject, url: &str) -> bool {
+        let Some(class) = AnyClass::get(c"SFSafariViewController") else {
+            return false;
+        };
+        let Some(url) = NSURL::URLWithString(&NSString::from_str(url)) else {
+            return false;
+        };
+
+        let allocated: Allocated<AnyObject> = msg_send![class, alloc];
+        let controller: Option<Retained<AnyObject>> = msg_send![allocated, initWithURL: &*url];
+        let Some(controller) = controller else {
+            return false;
+        };
+
+        let delegate: Retained<AnyObject> = msg_send![delegate_class(), new];
+        let () = msg_send![&*controller, setDelegate: &*delegate];
+        DELEGATE.with(|slot| *slot.borrow_mut() = Some(delegate));
+
+        let completion: Option<&DynBlock<dyn Fn()>> = None;
+        let () = msg_send![root, presentViewController: &*controller, animated: true, completion: completion];
+        PRESENTED.with(|slot| *slot.borrow_mut() = Some(controller));
+        true
+    }
+
+    pub fn dismiss(app: &tauri::AppHandle) {
+        let _ = app.run_on_main_thread(|| {
+            let Some(controller) = PRESENTED.with(|slot| slot.borrow_mut().take()) else {
+                return;
+            };
+            DELEGATE.with(|slot| slot.borrow_mut().take());
+            unsafe {
+                let completion: Option<&DynBlock<dyn Fn()>> = None;
+                // Sent to the presented controller rather than the presenting one, which UIKit
+                // forwards. Letting go of the last reference afterwards is safe: the presenting
+                // controller holds it for the length of the animation.
+                let () = msg_send![&*controller, dismissViewControllerAnimated: true, completion: completion];
+            }
+        });
+    }
+
+    // The second iOS surface, and the better one. See `authenticate` for what it buys.
+    #[link(name = "AuthenticationServices", kind = "framework")]
+    extern "C" {}
+
+    /// `ASWebAuthenticationSessionErrorCodeCanceledLogin`. The user closed the sheet, which is not
+    /// a failure worth dressing up as one.
+    const CANCELED_LOGIN: isize = 1;
+
+    thread_local! {
+        /// The session, the object that tells it which window to present over, and that window.
+        ///
+        /// A session nothing retains deallocates and then simply never calls back, which is the
+        /// classic way to lose an afternoon to this API, and the presentation context provider is
+        /// held weakly so it goes the same way. Replaced on the next attempt rather than cleared in
+        /// the completion handler, because the session owns the block that handler lives in and
+        /// releasing it from inside its own invocation is asking for a use after free.
+        static SESSION: RefCell<Option<Retained<AnyObject>>> = const { RefCell::new(None) };
+        static ANCHOR: RefCell<Option<Retained<AnyObject>>> = const { RefCell::new(None) };
+        static ANCHOR_WINDOW: RefCell<Option<Retained<AnyObject>>> = const { RefCell::new(None) };
+    }
+
+    /// Required from iOS 13 on: with no anchor the session refuses to start and reports
+    /// `presentationContextNotProvided` instead. Returned unretained, which is the convention for
+    /// a getter like this one, and safe because ANCHOR_WINDOW holds it for the flow.
+    extern "C-unwind" fn presentation_anchor(
+        _this: &AnyObject,
+        _cmd: Sel,
+        _session: *mut AnyObject,
+    ) -> *mut AnyObject {
+        ANCHOR_WINDOW.with(|slot| match slot.borrow().as_ref() {
+            Some(window) => Retained::as_ptr(window).cast_mut(),
+            None => std::ptr::null_mut(),
+        })
+    }
+
+    fn anchor_class() -> &'static AnyClass {
+        thread_local! {
+            static CLASS: &'static AnyClass = {
+                let mut builder = ClassBuilder::new(c"MarginConsentAnchor", NSObject::class())
+                    .expect("MarginConsentAnchor is registered once and by nobody else");
+                unsafe {
+                    builder.add_method(
+                        sel!(presentationAnchorForWebAuthenticationSession:),
+                        presentation_anchor as extern "C-unwind" fn(_, _, _) -> _,
+                    );
+                }
+                builder.register()
+            };
+        }
+        CLASS.with(|class| *class)
+    }
+
+    /// The consent page in an `ASWebAuthenticationSession`, which is the same Safari view underneath
+    /// but with Safari's own cookies rather than a jar of this app's alone. That is the whole point
+    /// of it: an account already signed in on this phone is offered by name instead of asking for a
+    /// password again. It is also why iOS puts up its own "wants to use google.com to sign in"
+    /// prompt first, since sharing the session is something the user gets to refuse.
+    ///
+    /// The price is that it only ever intercepts a custom scheme, never an http loopback redirect,
+    /// so this path exists only where a scheme exists: an `ios` block in the credentials file.
+    pub fn authenticate(app: &tauri::AppHandle, url: &str, scheme: &str) {
+        let Some(window) = app.get_webview_window("main") else {
+            return crate::google::auth::open_in_browser(app, url);
+        };
+        let inner_app = app.clone();
+        let inner_url = url.to_string();
+        let scheme = scheme.to_string();
+        let posted = window.with_webview(move |webview| {
+            let started = unsafe {
+                let view = webview.inner().cast::<AnyObject>();
+                let ui_window: Option<Retained<AnyObject>> = msg_send![view, window];
+                match ui_window {
+                    Some(ui_window) => start(&inner_app, ui_window, &inner_url, &scheme),
+                    None => false,
+                }
+            };
+            if !started {
+                crate::google::auth::open_in_browser(&inner_app, &inner_url);
+            }
+        });
+        if posted.is_err() {
+            crate::google::auth::open_in_browser(app, url);
+        }
+    }
+
+    unsafe fn start(
+        app: &tauri::AppHandle,
+        ui_window: Retained<AnyObject>,
+        url: &str,
+        scheme: &str,
+    ) -> bool {
+        let Some(class) = AnyClass::get(c"ASWebAuthenticationSession") else {
+            return false;
+        };
+        let Some(url) = NSURL::URLWithString(&NSString::from_str(url)) else {
+            return false;
+        };
+        let scheme = NSString::from_str(scheme);
+
+        let handler_app = app.clone();
+        let handler = RcBlock::new(move |callback: *mut NSURL, error: *mut NSError| {
+            finished(&handler_app, callback, error);
+        });
+
+        let allocated: Allocated<AnyObject> = msg_send![class, alloc];
+        let session: Option<Retained<AnyObject>> = msg_send![
+            allocated,
+            initWithURL: &*url,
+            callbackURLScheme: &*scheme,
+            completionHandler: &*handler,
+        ];
+        let Some(session) = session else {
+            return false;
+        };
+
+        // False, not true: an ephemeral session is a fresh cookie jar, which throws away the one
+        // reason to be using this API at all.
+        let () = msg_send![&*session, setPrefersEphemeralWebBrowserSession: false];
+
+        let anchor: Retained<AnyObject> = msg_send![anchor_class(), new];
+        let () = msg_send![&*session, setPresentationContextProvider: &*anchor];
+        ANCHOR_WINDOW.with(|slot| *slot.borrow_mut() = Some(ui_window));
+        ANCHOR.with(|slot| *slot.borrow_mut() = Some(anchor));
+
+        let started: bool = msg_send![&*session, start];
+        SESSION.with(|slot| *slot.borrow_mut() = Some(session));
+        started
+    }
+
+    /// One of three answers: the callback URL, a cancel, or a real failure. The URL goes to the same
+    /// `handle_redirect` the deep link uses, so the state check and the exchange stay in one place
+    /// and this knows nothing about either.
+    fn finished(app: &tauri::AppHandle, callback: *mut NSURL, error: *mut NSError) {
+        if !callback.is_null() {
+            let absolute = unsafe {
+                let string: Retained<NSString> = msg_send![callback, absoluteString];
+                string.to_string()
+            };
+            if let Ok(parsed) = url::Url::parse(&absolute) {
+                let app = app.clone();
+                tauri::async_runtime::spawn(async move {
+                    crate::google::auth::handle_redirect(app, &parsed).await;
+                });
+            }
+            return;
+        }
+
+        let code = if error.is_null() {
+            CANCELED_LOGIN
+        } else {
+            unsafe { msg_send![error, code] }
+        };
+        let reason = if code == CANCELED_LOGIN {
+            None
+        } else {
+            Some(format!("The sign-in sheet could not be shown (error {code})."))
+        };
+        let app = app.clone();
+        tauri::async_runtime::spawn(async move {
+            crate::google::auth::abandon_pending(app, reason).await;
+        });
+    }
+}
+
+/// Android has no equivalent of `with_webview`, and reaching a Custom Tab from here would mean JNI
+/// in Rust for the sake of one Intent. It goes through the JavaScript bridge in `MainActivity.kt`
+/// instead, which is the same shape as the window insets bridge that is already there.
+///
+/// That bridge is the one thing this depends on, and `tauri android init` regenerates the file it
+/// lives in. Without it the script below is a no-op and nothing opens at all, which is why
+/// docs/mobile.md says to check for it after running init.
+#[cfg(target_os = "android")]
+fn present(app: &tauri::AppHandle, url: &str) {
+    let literal = match serde_json::to_string(url) {
+        Ok(literal) => literal,
+        Err(_) => return crate::google::auth::open_in_browser(app, url),
+    };
+    if eval(app, &format!("window.__androidAuthTab?.open({literal})")).is_err() {
+        crate::google::auth::open_in_browser(app, url);
+    }
+}
+
+#[cfg(target_os = "android")]
+fn dismiss(app: &tauri::AppHandle) {
+    let _ = eval(app, "window.__androidAuthTab?.close()");
+}
+
+#[cfg(target_os = "android")]
+fn eval(app: &tauri::AppHandle, script: &str) -> Result<(), String> {
+    use tauri::Manager;
+
+    let window = app
+        .get_webview_window("main")
+        .ok_or("there is no window to run the bridge from")?;
+    window.eval(script).map_err(|e| e.to_string())
+}

src-tauri/src/google/mod.rs

@@ -1,9 +1,13 @@
-// auth.rs     OAuth, token refresh. Loopback on desktop, a deep link on mobile.
+// auth.rs     OAuth, token refresh. A loopback listener everywhere, a deep link when a phone has
+//             been given its own OAuth client.
+// browser.rs  the consent page in front of the app on a phone, and taking it away again
 // api.rs      typed Google Calendar REST wrapper
 // secrets.rs  refresh tokens, encrypted on disk, same on every platform
 
 pub mod api;
 pub mod auth;
+#[cfg(mobile)]
+pub mod browser;
 pub mod secrets;
 
 use crate::dto::Account;

src-tauri/src/lib.rs

@@ -29,7 +29,7 @@ fn build_menu<R: Runtime>(handle: &tauri::AppHandle<R>) -> tauri::Result<Menu<R>
     let sync_now = MenuItemBuilder::with_id("sync-now", "Sync Now")
         .accelerator("CmdOrCtrl+R")
         .build(handle)?;
-    let accounts = MenuItemBuilder::with_id("accounts", "Accounts…").build(handle)?;
+    let accounts = MenuItemBuilder::with_id("accounts", "Google Accounts…").build(handle)?;
     let check_updates =
         MenuItemBuilder::with_id("check-updates", "Check for Updates…").build(handle)?;
     let settings = MenuItemBuilder::with_id("settings", "Settings…")
@@ -136,8 +136,10 @@ fn build_menu<R: Runtime>(handle: &tauri::AppHandle<R>) -> tauri::Result<Menu<R>
     Ok(menu)
 }
 
-/// Google's answer to a consent request comes back as a link into this app rather than to a
-/// loopback port, because a phone has no loopback port to give it.
+/// The deep link half of mobile auth: Google's answer arrives as a link into this app rather than
+/// on a loopback socket. Only the flow that runs when a phone has its own OAuth client uses it, but
+/// it stays registered on both platforms either way, because the OS has to be told about a scheme
+/// at install time and cannot be told about one later.
 ///
 /// Both arrival routes are covered. `on_open_url` catches the link when the app was already
 /// running, which is the usual case since it is what opened the browser a moment ago;
@@ -196,6 +198,27 @@ fn stop_uikit_shrinking_the_viewport(window: &tauri::WebviewWindow) {
     });
 }
 
+/// A Chrome Custom Tab is Chrome's activity sitting on top of ours, in our own task, so this
+/// process really is backgrounded for the length of a consent round trip and coming back means the
+/// tab has gone. That is the only notice Android gives that the user backed out of signing in, and
+/// without it the accounts panel waits on an answer that is never coming. `note_resumed` works out
+/// whether the tab went because the user closed it or because this app took it down a moment after
+/// the code arrived.
+///
+/// Not `RunEvent::Resumed`, which sounds right and is not: tauri raises that one on every poll of
+/// the event loop. tao's real mobile resume arrives here, per window.
+///
+/// iOS shows the consent page inside the app, never leaves the foreground, and gets the same
+/// question answered by a delegate in `google/browser.rs` instead.
+#[cfg(target_os = "android")]
+fn watch_for_the_consent_tab_closing(window: &tauri::WebviewWindow) {
+    window.on_window_event(|event| {
+        if matches!(event, tauri::WindowEvent::Resumed) {
+            google::browser::note_resumed();
+        }
+    });
+}
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     // generate_context! first, so the updater plugin registers only when the merged config
@@ -230,6 +253,10 @@ pub fn run() {
             if let Some(window) = handle.get_webview_window("main") {
                 stop_uikit_shrinking_the_viewport(&window);
             }
+            #[cfg(target_os = "android")]
+            if let Some(window) = handle.get_webview_window("main") {
+                watch_for_the_consent_tab_closing(&window);
+            }
             Ok(())
         });