问题在于ScrollView和VStack的组合。默认情况下，ScrollView只占据其子视图所需的位置，这阻止了VStack的扩展。一种解决方案是使用GeomtryReader，如以下代码所示：

struct ContentView: View {
    var body: some View {
        GeometryReader { gm in  //----> add it here
            ScrollView {
                VStack {
                    HStack {
                        Text("Title")
                            .font(.largeTitle)
                            .fontWeight(.bold)
                            .padding(.leading)

                        Spacer()
                    }

                    Spacer()
                        .frame(height: 60)

                    HStack(alignment: .center) {
                        Text("Header Text")
                    }
                    .frame(height: 60)
                    .frame(maxWidth: .infinity)
                    .background(Color.white)
                    .cornerRadius(6.5)
                    .padding(.horizontal)
                    Spacer()
                        .frame(height: 60)

                    // I want this to occupy all the remaining space
                    VStack {
                        Text("ABC")

                        Text("DEF")
                    }
                    .frame(maxWidth: .infinity)
                    .frame(height: gm.size.height) // ----> use it here
                    .background(Color.yellow)
                }
                .frame(maxWidth: .infinity, maxHeight: .infinity)
            }
            .frame(maxWidth: .infinity, maxHeight: .infinity)
            .background(Color.green.ignoresSafeArea())
        }
    }
}