These modifiers are designed to observe changes in the view's bounds and provide a stretchy background effect for a list.

List+StretchyBackground.swift

import SwiftUI

/// These modifiers are designed to observe changes in the view's bounds and provide a stretchy background effect for a list.
///
/// Source: https://gist.github.com/satishVekariya/c52477b6acafdf200606335e39a37382
public extension View {
    /// List/View bound change observer
    ///
    ///  Mostly you need to use this on List cell
    ///
    ///  ```
    ///  @State private var offset: CGPoint?
    ///
    ///     List {
    ///         Text("hi")
    ///             .onBoundChange() {
    ///                 offset = $0?.origin
    ///             }
    ///     }
    ///     .stretchyBackground(offsetY: offset?.y)
    ///  ```
    /// Reference:  https://stackoverflow.com/questions/56726369/getting-content-offset-of-list-in-swiftui
    ///
    /// - Parameters:
    ///   - id: A unique id
    ///   - onChange: Closure to that receives new frame value
    /// - Returns: some View
    func onBoundChange(
        id: String = "bound",
        onChange: @escaping (CGRect?) -> Void
    ) -> some View {
        self.background(
            GeometryReader { geo in
                Color.clear
                    .preference(key: BoundPreferenceKey.self, value: [ViewFrame(id: id, frame: geo.frame(in: .named("name")))])
            }
        )
        .onPreferenceChange(BoundPreferenceKey.self) { frames in
            onChange(frames.first(where: { $0.id == id })?.frame)
        }
    }
    
    /// Use this modifier on List view to set fixed and stretchy background
    /// - Parameters:
    ///   - alignment: Alignment of the stretchy background
    ///   - fixedColor: Background color to fill entire screen. Default is white
    ///   - stretchyColor: Background color to display on top of fixed color with dynamic height
    ///   - offsetY: Scroll offset to adjust stretchy of stretchyColor. Mostly given by `onBoundChange(_:)` modifier
    /// - Returns: some View
    func stretchyBackground(
        alignment: Alignment = .top,
        fixedColor: Color = .white,
        stretchyColor: Color = .blue,
        offsetY: CGFloat?
    ) -> some View {
        self.background(
            ZStack(alignment: alignment) {
                fixedColor
                if let offsetY = offsetY, offsetY > 0 {
                    stretchyColor.frame(height: offsetY)
                }
            }
                .ignoresSafeArea()
                .coordinateSpace(name: "name")
        )
    }
}

fileprivate struct ViewFrame: Equatable {
    let id: String
    let frame: CGRect
    
    static func == (lhs: ViewFrame, rhs: ViewFrame) -> Bool {
        lhs.id == rhs.id && lhs.frame == rhs.frame
    }
}

fileprivate struct BoundPreferenceKey: PreferenceKey {
    typealias Value = [ViewFrame] // The list of view frame changes in a View tree.

    static var defaultValue: [ViewFrame] = []

    /// When traversing the view tree, Swift UI will use this function to collect all view frame changes.
    static func reduce(value: inout [ViewFrame], nextValue: () -> [ViewFrame]) {
        value.append(contentsOf: nextValue())
    }
}