How can I hide title bar in SwiftUI? (for MacOS app)

import SwiftUI
@main
struct TestApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }.windowStyle(HiddenTitleBarWindowStyle())
    }
}

try HiddenTitleBarWindowStyle()

Removing the Title Bar in Your Mac App Built with Mac Catalyst

Display content that fills the entire height of a window by removing the title bar.

By default, Mac apps built with Mac Catalyst display a title bar across the top of their windows. A horizontal line separates the title bar from the content of the window. Some Mac apps such as Messages and Contacts have no title bar in their main window. Instead, the top of the window shows only the Close, Minimize, and Zoom buttons with no separator between them and the window's content. In this UI design, the content area fills the entire height of the window. The following image illustrates these styles in two windows. The first window displays a title bar, while the second has none. Screenshot of two windows, one stacked above the other, with a dark background in the content area of each window.

Remove the Title Bar If you choose to design your window without a title bar, you must remove it from the window. To remove the title bar, set the title bar’s titleVisibility property to UITitlebarTitleVisibility.hidden and the toolbar property to nil. The following code shows how to remove the title bar and its separator from the window during the setup of a new scene.

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    
    guard let windowScene = (scene as? UIWindowScene) else { return }
    
    #if targetEnvironment(macCatalyst)
    if let titlebar = windowScene.titlebar {
        titlebar.titleVisibility = .hidden
        titlebar.toolbar = nil
    }
    #endif

}

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I could not find a way to hide the toolbar entirely in SwiftUI. But this is a possible workaround. You can put this code in your AppDelegate file.

func applicationDidFinishLaunching(_ aNotification: Notification) {
    let window = NSApplication.shared.windows.first!
    window.titlebarAppearsTransparent = true
    window.backgroundColor = .white
    window.standardWindowButton(.closeButton)!.isHidden = true
    window.standardWindowButton(.miniaturizeButton)!.isHidden = true
    window.standardWindowButton(.zoomButton)!.isHidden = true
}

Using this code will make it appear that the toolbar is hidden when in reality, it is still there. But the buttons are hidden, and the background is transparent.