How could I set the cornerRadius of a RoundedRectangle to be proportional to its size?

I have a RoundedRectangle that could get pretty big on large screens, i.e. iPads or MacOS windows, and in that case a fixed cornerRadius chosen for iPhone-size screens would seem inappropriate.

I've tried using a GeometryReader to get the current available width and multiply it by a certain scale factor to get the cornerRadius, this approach works, but the use of GeometryReader changes my layout (Probably because it takes up all the space available?). My current approach:

GeometryReader { geometry in
            let cornerRadius = geometry.size.width * 0.27
            RoundedRectangle(cornerRadius: cornerRadius
                .aspectRatio(5/7, contentMode: .fit)
        }

I've found that applying a .aspectRatio() modifier would solve the problem:

GeometryReader { geometry in
            let cornerRadius = geometry.size.width * 0.27
            RoundedRectangle(cornerRadius: cornerRadius)
                .aspectRatio(5/7, contentMode: .fit)
        }
        .aspectRatio(5/7, contentMode: .fit)

But this workaround doesn't seem proper since I probably wouldn't have a fixed aspect ratio for the content inside the next time I'm in this situation.

Without a GeometryReader: The original layout

With GeometryReader: You can see that the vertical spacing between rectangles are a bit wider

(I'm new to StackOverflow, so I can't directly put images here yet...)

Is there a cleaner approach to prevent GeometryReader from changing the layout or is there a cleaner way to make the cornerRadius proportional to the size of the RoundedRectangle? Any help would be greatly appreciated!

You could create your own Shape for this.

A Shape must implement a path function and this function is supplied with a CGRect. This gives us the dimensions of the area being drawn into:

struct ProportionalRoundedRectangle: Shape {
    let cornerFraction: CGFloat

    func path(in rect: CGRect) -> Path {
        let minSize = min(rect.width, rect.height)
        let cornerSize = min(0.5, max(0, cornerFraction)) * minSize
        return Path(
            roundedRect: rect,
            cornerSize: .init(width: cornerSize, height: cornerSize)
        )
    }
}

Example use:

VStack {
    ProportionalRoundedRectangle(cornerFraction: 0.1)
        .stroke(.blue, lineWidth: 3)
        .frame(width: 200, height: 100)
    ProportionalRoundedRectangle(cornerFraction: 0.25)
        .stroke(.red, lineWidth: 3)
        .frame(width: 200, height: 100)
        .overlay {
            ProportionalRoundedRectangle(cornerFraction: 0.4)
                .fill(.green)
                .padding()
        }
}