Pinning a view at the bottom of a scroll view

ScrollViewWithPinning.swift

import SwiftUI

struct ContentView: View {
    let itemHeight: CGFloat = 50
    
    var body: some View {
        GeometryReader { geo in
            ScrollView {
                VStack {
                    Text("Start")
                    Color.clear.frame(height: 800)
                    Text("End")
                    
                    Color.blue
                        .frame(height: itemHeight)
                        .anchorPreference(key: OffsetPreference.self, value: .bottom) { anchor in
                            max(geo[anchor].y, geo.size.height)
                        }
                }
            }
        }
        .overlayPreferenceValue(OffsetPreference.self, alignment: .top) { offset in
            Circle()
                .frame(width: itemHeight, height: itemHeight)
                .offset(y: offset - itemHeight)
        }
    }
}


enum OffsetPreference: PreferenceKey {
    static let defaultValue: CGFloat = 0
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {}
}

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