SwiftUI `.shadow` with spread parameters like css.

SpreadShadow.swift

import SwiftUI

struct SpreadShadow: ViewModifier {
  let color: Color
  let x: CGFloat
  let y: CGFloat
  let blur: CGFloat
  let spread: CGFloat

  init(color: Color, x: CGFloat = 0, y: CGFloat = 0, blur: CGFloat = 0, spread: CGFloat = 0) {
    self.color = color
    self.x = x
    self.y = y
    self.blur = blur
    self.spread = spread
  }

  func body(content: Content) -> some View {
    let composited = content.compositingGroup()
    return composited.background {
      GeometryReader { proxy in
        let spreadSize = CGSize(width: proxy.size.width + spread * 2, height: proxy.size.height + spread * 2)
        return Rectangle().fill(color)
          .frame(width: spreadSize.width, height: spreadSize.height)
          .mask(composited.drawingGroup().scaleEffect(spreadSize / proxy.size))
          .offset(x: x - spread, y: y - spread)
          .blur(radius: blur / 2)
          .overlay(alignment: .topLeading) {
            composited
              .frame(width: proxy.size.width, height: proxy.size.height)
              .blendMode(.destinationOut)
          }
          .compositingGroup()
      }
    }
  }
}

extension View {
  func spreadShadow(color: Color, x: CGFloat = 0, y: CGFloat = 0, blur: CGFloat = 0, spread: CGFloat = 0) -> some View {
    return modifier(SpreadShadow(color: color, x: x, y: y, blur: blur, spread: spread))
  }
}

#Preview {
  VStack(spacing: 40) {
    Button { } label: {
      Text("Button")
        .foregroundStyle(Color.black)
        .bold()
        .padding(20)
        .background(.white)
        .clipShape(Capsule())
        .spreadShadow(color: .black.opacity(0.5), y: 4, blur: 32, spread: -8)
        .rotationEffect(.init(degrees: 20))
    }

    Button { } label: {
      Text("Button")
        .foregroundStyle(Color.black)
        .bold()
        .padding(20)
        .background(.white)
        .clipShape(Capsule())
        .shadow(color: .black.opacity(0.5), radius: 16, y: 4)
        .rotationEffect(.init(degrees: 20))
    }
  }
}