"I need my layout to adapt..."
│
├─ TO AVAILABLE SPACE (container-driven)
│   │
│   ├─ "Pick best-fitting variant"
│   │   → ViewThatFits
│   │
│   ├─ "Animated switch between H↔V"
│   │   → AnyLayout + condition
│   │
│   ├─ "Read size for calculations"
│   │   → onGeometryChange (iOS 16+)
│   │
│   └─ "Custom layout algorithm"
│       → Layout protocol
│
├─ TO PLATFORM TRAITS
│   │
│   ├─ "Compact vs Regular width"
│   │   → horizontalSizeClass (⚠️ iPad limitations)
│   │
│   ├─ "Accessibility text size"
│   │   → dynamicTypeSize.isAccessibilitySize
│   │
│   └─ "Platform differences"
│       → #if os() / Environment
│
└─ TO WINDOW SHAPE (aspect ratio)
    │
    ├─ "Portrait vs Landscape semantics"
    │   → Geometry + custom threshold
    │
    ├─ "Auto show/hide columns"
    │   → NavigationSplitView (automatic in iOS 26)
    │
    └─ "Window lifecycle"
        → @Environment(\.scenePhase)

Do you need a calculated value (width, height)?
├─ YES → onGeometryChange
└─ NO → Do you need animated transitions?
         ├─ YES → AnyLayout + condition
         └─ NO → ViewThatFits

ViewThatFits {
    // First choice: horizontal
    HStack {
        Image(systemName: "star")
        Text("Favorite")
        Spacer()
        Button("Add") { }
    }

    // Fallback: vertical
    VStack {
        HStack {
            Image(systemName: "star")
            Text("Favorite")
        }
        Button("Add") { }
    }
}

struct AdaptiveStack<Content: View>: View {
    @Environment(\.horizontalSizeClass) var sizeClass

    let content: Content

    var layout: AnyLayout {
        sizeClass == .compact
            ? AnyLayout(VStackLayout(spacing: 12))
            : AnyLayout(HStackLayout(spacing: 20))
    }

    var body: some View {
        layout {
            content
        }
        .animation(.default, value: sizeClass)
    }
}

@Environment(\.dynamicTypeSize) var dynamicTypeSize

var layout: AnyLayout {
    dynamicTypeSize.isAccessibilitySize
        ? AnyLayout(VStackLayout())
        : AnyLayout(HStackLayout())
}

struct ResponsiveGrid: View {
    @State private var columnCount = 2

    var body: some View {
        LazyVGrid(columns: Array(repeating: GridItem(.flexible()), count: columnCount)) {
            ForEach(items) { item in
                ItemView(item: item)
            }
        }
        .onGeometryChange(for: Int.self) { proxy in
            max(1, Int(proxy.size.width / 150))
        } action: { newCount in
            columnCount = newCount
        }
    }
}

struct WindowShapeReader: View {
    @State private var isWide = true

    var body: some View {
        content
            .onGeometryChange(for: Bool.self) { proxy in
                proxy.size.width > proxy.size.height * 1.2
            } action: { newValue in
                isWide = newValue
            }
    }
}

// ✅ CORRECT: Constrained GeometryReader
VStack {
    GeometryReader { geo in
        Text("Width: \(geo.size.width)")
    }
    .frame(height: 44)  // MUST constrain!

    Button("Next") { }
}

// ❌ WRONG: Unconstrained (greedy)
VStack {
    GeometryReader { geo in
        Text("Width: \(geo.size.width)")
    }
    // Takes all available space, crushes siblings
    Button("Next") { }
}

// iOS 26: Columns automatically show/hide
NavigationSplitView {
    Sidebar()
} content: {
    ContentList()
} detail: {
    DetailView()
}
// No manual columnVisibility management needed

// ❌ WRONG: Reports device, not window
NotificationCenter.default.addObserver(
    forName: UIDevice.orientationDidChangeNotification, ...
)

let orientation = UIDevice.current.orientation
if orientation.isLandscape { ... }

// ❌ WRONG: Returns full screen, not your window
let width = UIScreen.main.bounds.width
if width > 700 { useWideLayout() }

// ❌ WRONG: Breaks on new devices, wrong in multitasking
if UIDevice.current.userInterfaceIdiom == .pad {
    useWideLayout()
}

// ❌ WRONG: GeometryReader is greedy
VStack {
    GeometryReader { geo in
        Text("Size: \(geo.size)")
    }
    Button("Next") { }  // Crushed
}

// ❌ WRONG: iPad is .regular in both orientations
var isLandscape: Bool {
    horizontalSizeClass == .regular  // Always true on iPad!
}