The difference is in the programming model. The library itself is probably quite easy to support both models. If you have some UI in retained mode, chances are

The difference is in the programming model. The library itself is probably quite easy to support both models. If you have some UI in retained mode, chances are that rendering it live every frame will lead to unacceptable performance. But if you have UI in immediate mode, you can probably switch to retained - essentially lazy rendering until something else happens. But you'll have to be very careful that your render calls are indeed pure functions, and cannot produce different outputs unless some input has indeed changed.