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.