1. Detect open vs closed with size classes

The cheapest pose check you can write. Outer display reports compact width; inner display reports regular width (iPad idiom).

struct ContentView: View {
    @Environment(\.horizontalSizeClass) private var hSizeClass

    var body: some View {
        if hSizeClass == .regular {
            DuoOpenView()    // inner display — two-column layout
        } else {
            DuoClosedView()  // outer display — single column
        }
    }
}

2. Never cache the pose — the fold is live

Opening or closing the Duo resizes your window while the app runs. Anything that snapshots the size class once will go stale mid-fold.

// ❌ Don't — goes stale when the user folds the device
@State private var isPad = UIDevice.current.userInterfaceIdiom == .pad

// ✅ Do — re-reads the live environment value every render
@Environment(\.horizontalSizeClass) private var hSizeClass
@Environment(\.verticalSizeClass) private var vSizeClass

3. Master-detail that adapts for free

NavigationSplitView collapses to a stack on compact width automatically — two columns open, one column closed, zero pose-specific code.

NavigationSplitView {
    SidebarView()
} detail: {
    DetailView()
}
// Open (regular width): side-by-side. Closed (compact): stacked. Done.

4. Keep controls off the hinge

The inner display folds at its horizontal center (~334 pt from either edge). Keep primary tap targets away from the middle strip — top and bottom are safe.

VStack(spacing: 0) {
    ToolbarView()      // top — safe
    Spacer()
    MainContent()
    Spacer()
    TabBarView()       // bottom — safe
}
// Avoid: a lone button centered exactly on the fold line.
// If content must span the fold, make it non-interactive (dividers, backgrounds).

5. Width threshold as a backup check

When you need points instead of size classes: outer is 466 pt wide, inner is 669 pt. A 550 pt threshold separates them with room to spare.

GeometryReader { geo in
    AdaptiveView(isOpen: geo.size.width > 550)
}
// 466 pt (closed) < 550 < 669 pt (open)

6. Preview both poses in Xcode 27.1

Pin previews to the exact Duo geometry so canvas and simulator agree. (The beta's Previews canvas also has a Display group for this.)

#Preview("Duo — Closed", traits: .fixedLayout(width: 466, height: 678)) {
    ContentView()
}

#Preview("Duo — Open", traits: .fixedLayout(width: 669, height: 951)) {
    ContentView()
}

7. Animate the fold transition

When the pose flips, SwiftUI re-renders with the new size class. Wrap the switch in an animation and state survives the fold instead of snapping.

struct ContentView: View {
    @Environment(\.horizontalSizeClass) private var hSizeClass

    var body: some View {
        Group {
            if hSizeClass == .regular {
                DuoOpenView()
            } else {
                DuoClosedView()
            }
        }
        .animation(.smooth, value: hSizeClass)
    }
}

Keep going

Exact points, pixels and scale factors: device specs. Convert them: pt/px converter. Everything to verify before launch: the adaptation checklist. Frame a screenshot for your pitch deck: the mockup generator.