For anyone curious, you can now just set the MaximumHeight, MinimumHeight, MaximumWidth, and MinimumWidth to a static value so that they won't change. something like this:

        protected override Window CreateWindow(IActivationState? activationState)
        {
            const int newWidth = 1280;
            const int newHeight = 640;
            var window = new Window(new AppShell())
            {
                Width = newWidth,
                Height = newHeight
            };

            window.MaximumHeight = newHeight;
            window.MinimumHeight = newHeight;
            window.MaximumWidth = newWidth;
            window.MinimumWidth = newWidth;
            
            return window;
        }