Brushed Metal shader effect for SwiftUI

brushed.metal

#include <metal_stdlib>
#include <SwiftUI/SwiftUI_Metal.h>
using namespace metal;

// use Color(0.73,0.73,0.73,1.0) as currentColor for the original brushed look

// draw long bright sine waves on current color
// add up phase in each line to not have the same curve in each line but overlapping brush grooves
[[ stitchable ]] half4 brushedMetal(float2 position, half4 currentColor, float2 size) {
    float phaseOffset = cos((position.y*size.x + position.x) / size.x);
    float pattern = sin(position.y/size.y + phaseOffset * 12)
                    + cos(sin(position.y / size.y));
    half4 highlightColor = currentColor+half4(0.12,0.12,0.12,-0.33);
    half4 mixed = mix(currentColor, highlightColor, pattern);
    return half4(((1 - mixed) * highlightColor + mixed * currentColor - sin(position.x/size.x*M_PI_F - M_PI_F)*0.4-0.2).rgb, currentColor.a);
}

BrushedMetalDemoApp.swift

import SwiftUI

@main
struct BrushedMetalDemoApp: App {
    var body: some Scene {
        WindowGroup {
            BrushedMetalView {
                ContentView()
            }
        }
        .windowStyle(.hiddenTitleBar)
        .windowToolbarStyle(.unified)
    }
}

ContentView.swift

import SwiftUI

struct BrushedMetalView<Content: View>: View {
    var content: () -> Content
    
    var body: some View {
        GeometryReader { geometry in
            content()
                .background{
                    Rectangle()
                        .ignoresSafeArea()
                        .colorEffect(ShaderLibrary.brushedMetal(.float2(geometry.size)))
                }
        }
    }
}

struct ContentView: View {
    @State var text: String = ""
    var body: some View {
        VStack {
            TextEditor(text: $text)
                .padding()
        }
        .background(Color(cgColor: NSColor.textBackgroundColor.cgColor))
        .cornerRadius(8)
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .padding()
        .shadow(color:.white,radius: 0,y:1)
        .shadow(color:.black,radius: 0,y:-1)
    }
}

#Preview {
    BrushedMetalView {
        ContentView()
    }
}

Preview on how it looks like.:

struct BrassMetalModifier: ViewModifier {

  static let darkWindowBackgroundColor =
    CGColor(gray: 50/256, alpha: 1.0)
  
  func body(content: Content) -> some View {
    content.background {
      Color(Self.darkWindowBackgroundColor) // effect works on this
        .overlay {
          GeometryReader { geometry in
            Color.black // no effect??
              .colorEffect(ShaderLibrary.brushedMetal(.float2(geometry.size)))
          }
        }
    }
  }
}
extension View {
  func brushedMetal() -> some View {
    modifier(BrassMetalModifier())
  }
}