Supporting IMGUI
Use the IMGUIContainer to place IMGUI code inside of a VisualElement. Everything you can normally do inside of OnGUI() is supported.
You can arrange multiple IMGUIContainer and lay them out by mixing GUILayout and UI Toolkit layouts. Note that it is not possible to add VisualElement instances inside of an IMGUIContainer.
IMGUIContainer example- 2018–11–02 Page amended