UMG’s Retainer Box allows you to detach widget render frequency from the main game render so you can update the widget less often than the current framerate. Since it outputs to a render target, as a side effect, it gives you a chance to add some basic post process via material.
Are you sure Retainer Box is what you need?
Perhaps you’re looking for a Scrollbox - that’s the component that automatically handles children larger than its original size.