Make a VStack fill the width of the screen in SwiftUI

Try using the .frame modifier with the following options:

.frame(
      minWidth: 0,
      maxWidth: .infinity,
      minHeight: 0,
      maxHeight: .infinity,
      alignment: .topLeading
    )

struct ContentView: View {
  var body: some View {
    VStack(alignment: .leading) {
      Text("Hello World")
        .font(.title)
      Text("Another")
        .font(.body)
      Spacer()
    }
    .frame(
      minWidth: 0,
      maxWidth: .infinity,
      minHeight: 0,
      maxHeight: .infinity,
      alignment: .topLeading
    )
    .background(Color.red)
  }
}

This is described as being a flexible frame (see the documentation), which will stretch to fill the whole screen, and when it has extra space it will center its contents inside of it.