ContentView.swift

//
//  ContentView.swift
//  NSWindowResizingQuestion
//
//  Created by William Power on 11/14/24.
//

//This project was created to support answering a question from:
// https://stackoverflow.com/questions/79170528/nswindow-with-swiftui-content-has-zero-frame-on-macos-15

import SwiftUI
import AppKit

struct ContentView: View {
    var body: some View {
        VStack {
            Button(action:launchWelcomeView){
                Label("Launch Welcome View", systemImage: "arrow.2.circlepath.circle")
            }
        }
        .padding()
    }
}

extension ContentView {
    private func launchWelcomeView() {
        let otherView = WelcomeView()
        
        let hostVC = NSHostingController(rootView: otherView)
        
        let window = NSWindow(contentViewController: hostVC)
        window.toolbar = NSToolbar()
        window.makeKeyAndOrderFront(self)
        
        NSApplication.shared.mainWindow?.windowController?.showWindow(window)
    }
}

#Preview {
    ContentView()
}

NSWindowResizingQuestionApp.swift

//This project was created to support answering a question from:
// https://stackoverflow.com/questions/79170528/nswindow-with-swiftui-content-has-zero-frame-on-macos-15

@main
struct NSWindowResizingQuestionApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

WelcomeView.swift

//
//  WelcomeView.swift
//  NSWindowResizingQuestion
//
//  Created by William Power on 11/14/24.
//
//This project was created to support answering a question from:
// https://stackoverflow.com/questions/79170528/nswindow-with-swiftui-content-has-zero-frame-on-macos-15

import SwiftUI

struct WelcomeView: View {
    var body: some View {
        ZStack(alignment: .top) {
            Text("Hello, SO")
        }
        //the following 2 properties set the window to a particular size
        .frame(width: 600, height: 400)
        .fixedSize()
        .background(Color.red)
    }
}

#Preview {
    WelcomeView()
}