one.swift

//

import SwiftUI

struct ContentView: View {
    @State private var toggle = true
    @Namespace private var ns
    var body: some View {
        let box = Rectangle()
            .fill(.tertiary)
            .frame(width: 100, height: 100)
        let circle = Circle()
            .fill(.blue)
            .overlay { Text("1").font(.title) }
            .transition(.offset()) // no-op because .identity doesn't do what you'd expect
            .matchedGeometryEffect(id: "id", in: ns)
        Grid {
            GridRow {
                box
                    .overlay {
                        if toggle {
                            circle
                                .padding()
                        }
                    }
                box
                    .overlay {
                        if !toggle {
                            circle
                                .padding()
                        }
                    }
            }
            GridRow {
                box
                box
            }
        }
        .animation(.default.speed(0.1), value: toggle)
        .contentShape(Rectangle())
        .onTapGesture {
            toggle.toggle()
        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

two.swift

//

import SwiftUI

struct ContentView: View {
    @State private var toggle = true
    @Namespace private var ns
    var body: some View {
        let box = Rectangle()
            .fill(.tertiary)
            .frame(width: 100, height: 100)
        let circle = Circle()
            .fill(.blue)
            .overlay { Text("1").font(.title) }
        Grid {
            GridRow {
                box
                    .matchedGeometryEffect(id: "box0", in: ns)
                box
                    .matchedGeometryEffect(id: "box1", in: ns)
            }
            GridRow {
                box
                box
            }
        }
        .overlay {
            circle
                .padding()
                .matchedGeometryEffect(id: toggle ? "box0" : "box1", in: ns, isSource: false)
        }
        .animation(.default.speed(0.1), value: toggle)
        .contentShape(Rectangle())
        .onTapGesture {
            toggle.toggle()
        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}