I'm not a fan of IMGUI, but there is nothing about it that prevents caching data (in fact AFAIK most IMGUI libraries do cache data), so it'd be done pretty much

I'm not a fan of IMGUI, but there is nothing about it that prevents caching data (in fact AFAIK most IMGUI libraries do cache data), so it'd be done pretty much the same way as in a regular GUI library: by caching the (unscrolled) item rects, updating the rect cache whenever the listbox size changes, caching the visible (after scrolling) items and updating the cache whenever the listbox size or scroll position change. Might have forgotten some other case, but the idea is the same.