Make ScrollView content fill its parent in SwiftUI

A simple way for you, using frame(maxWidth: .infinity)

    ScrollView(.vertical) {
        VStack {
           ForEach(0..<100) {
              Text("Item \($0)")
          }
       }
       .frame(maxWidth: .infinity)
   }

Actually you don't need GeometryReader anymore. ScrollView has been refactored in Xcode beta 3. Now you can declare that you have a .horizontal or .vertical ScrollView. This makes the ScrollView behave like it should, like any normal View protocol.

Ex:

ScrollView(.horizontal, showsIndicators: false) {
    HStack {
        ForEach((1...10).reversed()) {
            AnyViewYouWant(number: $0)
        }
    }
}

This will result in a view with the width of its parent scrolling horizontally. The height will be defined by the ScrollView's subviews height.