EditorGUILayout.Popup method does not work for EditorWindows that are called with window.ShowModal() for MacOS platform. To reproduce the bug, you have to use MacOS: 1. Create custom EditorWindow. 2. Add EditorGUILayout.Popup method to the OnGUI. 2. Open window with window.ShowModal() method. 3. Try to change value for the popup. It will be grayed out.