ContentView.swift

import SwiftUI

struct ContentView: View {
    @State private var initOffset: CGFloat = .zero
    @State private var offset: CGFloat = .zero
    @State private var closeButtonYPosition: CGFloat = .zero
    
    let headerHeight: CGFloat = 300
    
    var body: some View {
        
        ZStack(alignment: .topTrailing) {
            ScrollView {
                ZStack {
                    VStack(spacing: 0) {
                        ForEach(0..<50, id: \.self) { i in
                            Text("item \(i)")
                                .frame(maxWidth: .infinity, minHeight: 44)
                                .background(.white)
                        }
                    }
                    .padding(.top, headerHeight)
                    
                    GeometryReader { gr in
                        Color.cyan
                            .frame(height: headerHeight)
                            .overlay {
                                VStack {
                                    Text("Header")
                                }
                            }
                            .onAppear {
                                initOffset = gr.frame(in: .global).origin.y
                            }
                            .offset(y: gr.frame(in: .global).origin.y < initOffset
                                    ? 0
                                    : initOffset - gr.frame(in: .global).origin.y)
                            .onChange(of: gr.frame(in: .global).origin.y, initial: true) { _, newValue in
                                offset = gr.frame(in: .global).minY
                            }
                    }
                }
            }
            .background(.gray)
            
            GeometryReader { gr in
                HStack {
                    Spacer()
                    Button {
                        print("close")
                    } label: {
                        Image(systemName: "xmark.circle.fill")
                            .resizable()
                            .frame(width: 34, height: 34)
                            .foregroundStyle(headerHeight + offset - initOffset + 17 < closeButtonYPosition ? .green : .red)
                            .background(.white)
                            .clipShape(Circle())
                    }
                    .frame(width: 44, height: 44)
                    .padding()
                    .onAppear {
                        closeButtonYPosition = gr.frame(in: .global).origin.y
                    }
                }
            }
        }
    }
}

#Preview {
    ContentView()
}