Window scenes: several windows of one app (MOB-245).

On iPad every window of an app is a scene, and an app can show several side by side (Split View, Stage Manager) or switch between them. In a mob app they all run in the one BEAM, and each has its own router, navigation and screens: the window the app started in runs the root screen from Mob.Screen.start_root/2, and every further window starts on that same screen with the same params, then navigates on its own.

Opting in

iOS shows one window per app unless Info.plist says otherwise. Set it in mob.exs:

config :mob_dev, multi_window: true

mob_dev writes UIApplicationSupportsMultipleScenes into the built app's Info.plist (dev builds and mix mob.release). Rebuild natively after changing it.

Opening a window

The user can always open one from the app switcher or by dragging. To offer a button, show it only where it can work. supported?/0 asks the main thread, and the answer never changes while the app runs, so read it once in mount/3, not in render/1:

def mount(_params, _session, socket) do
  {:ok, Mob.Socket.assign(socket, :multi_window, Mob.Scene.supported?())}
end

def render(assigns) do
  ~MOB"""
  <Column>
    <Button :if={@multi_window} text="New window" on_tap={{self(), :new_window}} />
  </Column>
  """
end

def handle_info({:tap, :new_window}, socket) do
  Mob.Scene.request_new()
  {:noreply, socket}
end

# The system can still refuse, e.g. on iPhone Duo's outer display.
def handle_info({:mob_scene, :request_failed, _reason}, socket), do: {:noreply, socket}

What each window gets

• Its own navigation: push_screen/2 and friends act on the window the screen is in, and so does the back gesture.
• Its own :safe_area and :size_class assigns: two windows side by side are often different classes.
• Alerts and action sheets (Mob.Alert) shown by a screen appear in that screen's window, and their results come back to it.
• Mob.WebView calls and Mob.Keyboard.dismiss/1 act on the screen's own window, WebView messages come back to it, and the back gesture steps only that window's WebView history.

Shared by every window: the app's processes, Mob.State, persisted screen state (one Mob.ScreenState entry per screen module, last write wins), and notifications, which go to the window the app started in unless a process registered for them.

Not yet per window: the share sheet (Mob.Share), the file picker and toasts still pick a window themselves (the front or the first window), so one opened from a second window can appear in another. A picked file still reaches the calling screen (MOB-503).

A window the user closes, or one iPadOS discards in the background to save memory, has its screens stopped (persisted screens dump their state); if it comes back it starts on the root screen again. The last window is never stopped, so a single-window app keeps its state as it always has.

Mob.Test.screens/1 lists what every window shows, and Mob.Test helpers take scene: to address one. See decisions/2026-10-02-one-router-per-window-scene.md.