Toggle game window maximized state (ctrl+M).

EditorMaximizeGameWindow.cs

using UnityEditor;
using System.Reflection;

namespace FunktronicLabs
{
#if UNITY_EDITOR
    public class EditorMaximizeGameWindow
    {
        [MenuItem("Tools/Maximize Game Window %M")]
        public static void MaximizeGameWindow()
        {
            var assembly = Assembly.GetAssembly(typeof(Editor));
            var type = assembly.GetType("UnityEditor.GameView");
            var property = type.GetProperty("maximized");
            var window = EditorWindow.GetWindow(type);
            var maximized = (bool)property.GetValue(window, null);
            property.SetValue(window, (object)(!maximized), null);
        }
    }
#endif
}

Thank you, but It does not work on undocked window