4.6 UI sort order in fornt of IMGUI/OnGUI possible?

Yes you can draw the new UI on top of the old IMGUI. You just need to setup your new UI properly and render it manually from inside OnGUI.

The repaint event of the OnGUI callback is pretty much the last thing that is rendered. It will fire when all cameras have finished rendering. That's why you have to do the following:

• Setup your canvas to be "screenspace - Camera" and create a dedicated UI camera for this canvas. You might want to use a seperate layer for that UI stuff and set the camera cullingmask accordingly.

• Make sure you assign that camera in your UI canvas.

• Make sure you disable the camera and set it's clearflags to "Depth only"

Inside OnGUI you can now manually render your UI camera after your other OnGUI stuff::

 public Camera UICamera;
 void Start()
 {
     UICamera.enabled = false;
 }
 void OnGUI ()
 {
     if (GUI.Button(new Rect(10,10,300,200),"Test Button"))
     {

     }
     if (Event.current.type == EventType.Repaint)
     {
         UICamera.Render(); // this will render the new UI
     }
 }
 

Note that if you need interactive elements in both UIs you have to take care yourself of which should get the input. The new UI uses the new Eventsystem while OnGUI directly processes input usually around the time when Update is called.

Though pure visual UI should work just fine that way.

If you have multiple scripts that use OnGUI and you want the new UI on top of all of them you simply would render the camera from a seperate script with OnGUI callback that is ensured to run last / is on top.