> A lot of immediate mode "scroll view" systems just have a child lambda passed with an index for which element to render

This sounds a lot like paintEvent() in traditional OO-style GUI systems; i.e. event-driven.

So my understanding now is that with immediate-mode callbacks happen within the scope of a function-call rather than from some event-loop. I probably have to look into it deeper to get a good understanding. It is still unclear where state is stored (e.g. for the position of a scroll-view), and if state is passed through the function call tree on every frame.