A simple application to show a black, resizable, title-less, always-on-top NSWindow on Mac.

FloatingOverlayApp.swift

//
//  FloatingOverlayApp.swift
//  FloatingOverlay
//
//  Created by JuniperPhoton on 2025/3/12.
//

import SwiftUI

// Dependency: https://github.com/sindresorhus/KeyboardShortcuts
import KeyboardShortcuts

extension KeyboardShortcuts.Name {
    static let toggleWindow = Self("ToggleWindow", default: .init(.l, modifiers: [.command, .option]))
}

class AppDelegate: NSObject, NSApplicationDelegate {
    func applicationDidFinishLaunching(_ notification: Notification) {
        guard let window = NSApplication.shared.windows.first else {
            return
        }
        
        window.configureWindow()
        
        // If users maximum a NSWindow to the fullscreen mode, we need to activate it using keyboard shortcuts.
        // This [library](https://github.com/sindresorhus/KeyboardShortcuts) can help us implement this feature.
        // Also, make sure to set "Application is agent (UIElement)" to true in Info.plist.
        KeyboardShortcuts.onKeyUp(for: .toggleWindow) {
            if let window = NSApplication.shared.windows.first {
                // Make sure to reconfigure the window first.
                window.configureWindow()
                NSApplication.shared.activate()
            }
        }
    }
}

extension NSWindow {
    func configureWindow() {
        self.titleVisibility = .hidden
        self.titlebarAppearsTransparent = true
        self.isMovableByWindowBackground = true
        self.level = .screenSaver
        self.styleMask = [.borderless, .resizable]
        self.collectionBehavior = [
            .fullScreenAuxiliary,
            .stationary,
            .moveToActiveSpace
        ]
    }
}

@main
struct FloatingOverlayApp: App {
    @NSApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
    
    var body: some Scene {
        WindowGroup {
            ContentView().preferredColorScheme(.dark)
        }
    }
}

struct ContentView: View {
    var body: some View {
        VStack {
            // empty
        }
        .padding()
        .background {
            Rectangle().fill(.clear)
        }
    }
}