Header

Header.swift

// https://twitter.com/Kyomesuke/status/1702700847183151125

import SwiftUI

struct BiggerWidthPreferenceKey: PreferenceKey {
    static var defaultValue: CGFloat { .zero }

    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value = max(value, nextValue())
    }
}

struct ContentView: View {
    @State var buttonWidth: CGFloat?
    
    var body: some View {
        HStack {
            Button("Cancel Cancel") {}
                .fixedSize()
                .frame(maxWidth: .infinity, alignment: .leading)
                .background {
                    GeometryReader { proxy in
                        Color.clear.preference(
                            key: BiggerWidthPreferenceKey.self,
                            value: proxy.size.width
                        )
                    }
                }
                .frame(minWidth: buttonWidth)

            Text("TitleTitleTitleTitleTitleTitle")
                .frame(maxWidth: .infinity)
                .layoutPriority(1)

            Button("Done") {}
                .fixedSize()
                .frame(maxWidth: .infinity, alignment: .trailing)
                .background {
                    GeometryReader { proxy in
                        Color.clear.preference(
                            key: BiggerWidthPreferenceKey.self,
                            value: proxy.size.width
                        )
                    }
                }
                .frame(minWidth: buttonWidth)
        }
        .lineLimit(1)
        .onPreferenceChange(BiggerWidthPreferenceKey.self) { width in
            buttonWidth = width
        }
        .background {
            Color.red.frame(width: 1, height: 100)
        }
    }
}

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