App.xaml.cs

namespace PCCE
{
    public partial class App : Application
    {
        const int WindowWidth = 800;
        const int WindowHeight = 800;

        public App()
        {
            InitializeComponent();

#if WINDOWS
            Microsoft.Maui.Handlers.WindowHandler.Mapper.AppendToMapping(nameof(IWindow), (handler, view) =>
            {

                var mauiWindow = handler.VirtualView;
                var nativeWindow = handler.PlatformView;
                nativeWindow.Activate();
                IntPtr windowHandle = WinRT.Interop.WindowNative.GetWindowHandle(nativeWindow);
                var windowId = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(windowHandle);
                var appWindow = Microsoft.UI.Windowing.AppWindow.GetFromWindowId(windowId);
                appWindow.Resize(new Windows.Graphics.SizeInt32(WindowWidth, WindowHeight));
            });
#endif
        }

        protected override Window CreateWindow(IActivationState? activationState)
        {
            return new Window(new AppShell());
        }
    }
}