Capacitor on iPhone Duo

Start here, because it is the thing that will waste your afternoon otherwise:

WebKit 27.1 implements Device Posture and Viewport Segments, but both sit behind an experimental flag that only Safari can set.

A Capacitor app runs in a WKWebView it does not control the flags of, so navigator.devicePosture, the device-posture media query and the env(viewport-segment-*) variables are all absent there, while the same page in Safari on the same device can see them. Feature-detect rather than assume, and plan for both to be missing.

So the web platform gives you no fold awareness on this device out of the box. What it does give you is most of what matters. Work in two tiers.

Tier 1 — works today, no native code

Trust safe areas, and never assume symmetry

This is the one that produces real bugs. iOS moves its bars to one side of the display, and that side depends on the pose and the reading direction.

The bar arrives as a genuine safe-area inset: measured on the 27.1 simulator it is env(safe-area-inset-right) of 84px, and it moves to env(safe-area-inset-top) of 82px if the app opts out of vertical bars, live and without a reload. So most layouts need nothing more than honest env(safe-area-inset-*) padding on all four edges.

Any expression of the form width - env(safe-area-inset-left) * 2 is a defect on this device.

Resize correctly

The window changes size constantly: opening, closing, folding, rotating, Split View. Read sizes when you need them rather than caching, and prefer container queries and %/fr layout over stored pixel values. Anything measured in a useEffect or ngOnInit and kept will be stale within seconds.

Think in size classes, not device checks

iOS reasons about this device in size classes: the outer display is compact width, the inner display is regular width. Branch on the size you were given, never on a user-agent string or an orientation, since the inner display does not honour supported orientations.

Measured on the simulator: inner display 951×669 in landscape, outer display 466×678.

Move navigation to the side on the wide display

When iOS moves its own tab bars and toolbars to a vertical rail, an HTML tab bar pinned to the bottom looks unconverted next to every native app. A side rail on the inner display and a bottom bar on the outer one is the web analogue, and it costs one breakpoint.

Tier 2 — needs the native side

Everything genuinely iPhone Duo-specific is invisible to the web view until something native hands it over.

Two details worth knowing before you write any of this yourself. A flat phone's fold region is reported inactive, so reading it needs options: .includeInactive or you will conclude the device has no hinge. And UIHinge.status updates lazily, so it disagrees with the angle mid-fold; treat the angle as the truth when both are available.

Use the existing bridge

@erkamyaman/capacitor-foldable is an MIT plugin that already does this, and also fills in Device Posture and Viewport Segments so the standard CSS works in an app today:

npm install @erkamyaman/capacitor-foldable
npx cap sync

import { Foldable, installFoldablePolyfill } from '@erkamyaman/capacitor-foldable';

await installFoldablePolyfill();

const fold = await Foldable.getFoldState();
await Foldable.addListener('foldStateChange', (state) => layout(state));

The polyfill sets --fold-left, --fold-width and friends on <html>, so a grid can put its gutter exactly on the crease rather than guessing. The same build covers Android foldables through androidx.window.

Deeper material lives in iphone-duo-capacitor-skills: the CSS patterns, the full API, Ionic specifics, and how to test on the simulator and on an Android emulator.

Ionic apps

Ionic's ion-tabs is HTML, so it stays at the bottom while native bars move. Two routes: a stylesheet that turns the tab bar into a side pill on the edge iOS chose, or @rdlabo/capacitor-native-ui, which renders existing ion-buttons, ion-button, ion-back-button and ion-tab-bar with UIKit and SwiftUI while the originals, their state and their handlers stay in the web view. It takes device state as input, so the two compose rather than compete, and it lists the plugin in its own setup. The optional @rdlabo/ionic-theme-ios27 adds iOS 27 styling on top.

What to do, in order

1. Audit every safe-area expression for assumed symmetry. Most bugs die here.
2. Stop caching sizes, and branch on the size you were handed.
3. Give the wide display a real layout rather than a stretched phone one.
4. Add the plugin, and feature-detect the standard APIs so the same build still works in a browser that has them.
5. Only then reach for hinge angle effects, which are the smallest part of a good port.

Steps 1 to 3 need no native code and improve the app on tablets at the same time. Step 4 is the only iPhone Duo-specific part, and it cannot be tested without the Xcode 27.1 simulator.

For what the native APIs do and why, read ../iphone-duo-adaptive-layout/SKILL.md and ../iphone-duo-vertical-bars/SKILL.md — the design reasoning transfers directly even though the code does not.