Provides extensions to get called when a ScrollView is scrolled. First use .trackScrolling on the ScrollView, then put a ScrollViewReader inside. On the ScrollViewReader use the .onScrolled extension to receive the events (one argument, a UnitPoint).

CaptureScrollingOffset.swift

import SwiftUI


public extension ScrollView {

    // NB: CaptureScrollingOffset.EnvironmentKey is a class, so the value can be "shared" between Views,
    //     allowing both GeometryReader and Environment to participate in the same view.
    func trackScrolling() -> some View {
        let coordinateSpace = UUID()
        let environmentKey = CaptureScrollingOffset.EnvironmentKey(coordinateSpace: coordinateSpace)

        return self
            .coordinateSpace(name: coordinateSpace)
            .background(
                GeometryReader { geometry -> Color in
                    environmentKey.bounds = geometry.frame(in: .local)
                    return Color.clear
                }
            )
            .environment(\.captureScrollingOffset, environmentKey)
    }
    
}


public extension ScrollViewReader {

    func onScrolled(_ perform: @escaping (UnitPoint) -> Void) -> some View {
        return ModifiedContent(content: self, modifier: CaptureOffsetViewModifier())
            .onPreferenceChange(CaptureScrollingOffset.PreferenceKey.self) {
                var x: CGFloat = $0.content.origin.x
                var y: CGFloat = $0.content.origin.y

                if $0.content.width != $0.bounds.width {
                    x = x / ($0.content.width - $0.bounds.width)
                }

                if $0.content.height != $0.bounds.height {
                    y = y / ($0.content.height - $0.bounds.height)
                }

                x = (x * 10000).rounded() / 10000
                y = (y * 10000).rounded() / 10000

                x = min(max(x, 0), 1)
                y = min(max(y, 0), 1)

                perform(UnitPoint(x: x, y: -y))
            }
    }
}


fileprivate struct CaptureOffsetViewModifier: ViewModifier {

    @Environment(\.captureScrollingOffset) private var captureScrollingOffset

    func body(content: Content) -> some View {
        assert(captureScrollingOffset != nil, "Unexpected state, coordinate space not set for tracking offset changes")
        return captureScrollingOffset == nil ? ViewBuilder.buildEither(first: content) :
            ViewBuilder.buildEither(second: content.background(
                GeometryReader { geometry in
                    return Color.clear.preference(key: CaptureScrollingOffset.PreferenceKey.self,
                                                  value: CaptureScrollingOffset.KeyData(bounds: captureScrollingOffset!.bounds, content: geometry.frame(in: .named(captureScrollingOffset!.coordinateSpace))))
                }
            ))
    }
    
}


fileprivate struct CaptureScrollingOffset {

    class EnvironmentKey: SwiftUI.EnvironmentKey {
        let coordinateSpace: AnyHashable
        var bounds: CGRect

        static var defaultValue: CaptureScrollingOffset.EnvironmentKey? = nil

        init(coordinateSpace: AnyHashable, bounds: CGRect = .zero) {
            self.coordinateSpace = coordinateSpace
            self.bounds = bounds
        }

    }

    struct KeyData: Equatable {
        
        static var `default` = KeyData(bounds: .zero, content: .zero)
        
        let bounds: CGRect
        let content: CGRect

        static func == (lhs: CaptureScrollingOffset.KeyData, rhs: CaptureScrollingOffset.KeyData) -> Bool {
            return lhs.bounds == rhs.bounds && lhs.content == rhs.content
        }
    }

    struct PreferenceKey: SwiftUI.PreferenceKey {
        static var defaultValue: CaptureScrollingOffset.KeyData = .default

        static func reduce(value: inout CaptureScrollingOffset.KeyData, nextValue: () -> CaptureScrollingOffset.KeyData) {
        }
    }
    
}


fileprivate extension EnvironmentValues {
    
    var captureScrollingOffset: CaptureScrollingOffset.EnvironmentKey? {
        get { self[CaptureScrollingOffset.EnvironmentKey.self] }
        set { self[CaptureScrollingOffset.EnvironmentKey.self] = newValue }
    }

}