Anchor a portion of a SwiftUI view during animation in an NSWindow

Question

In the following code, I have a simple button that toggles the appearance of a rectangle between two elements in a VStack.

When this toggle happens, the enclosing NSWindow changes size immediately and then the contents changes with the animation.

A side effect is that the "Hello, World" Text appears to move in and out of place (where I would hope that the Text would stay anchored to the top).

Is there a way that I can do the following (with animation) where the Text element stays anchored at the top of the View and doesn't move while the rectangle comes into place and the Button slides down/up to accommodate the rectangle?

struct ContentView: View {
    var body: some View {
        Button("Show window") {
            let window = NSWindow()
            
            window.styleMask = [.titled, .closable]
            
            window.contentView = NSHostingView(rootView: WindowContent())
            
            window.makeKeyAndOrderFront(nil)
            
            window.center()
        }
        .padding()
    }
}

struct WindowContent: View {
    @State private var showRectangle = false
    
    var body: some View {
        VStack {
            Text("Hello, world")
            
            if showRectangle {
                Rectangle().fill(Color.red)
                    .frame(height: 100)
                    .frame(maxWidth: .infinity)
            }
            
            Button(action: {
                withAnimation(.linear(duration: 3)) {
                    showRectangle.toggle()
                }
            }) {
                Text("Toggle")
            }
        }
        
            .fixedSize()
            .padding(20)
    }
}