Yeah, it is understandably hard to visualize. In my head it was underneath/in-between regular replies. I could see it working in a couple different ways though… I put together some mockups:
3 different possible approaches to an in-line expandable chat. Interaction varies between the 3. Maybe on that last one you just click the speech bubble with the number (count of chat messages) to expand and/or click the space in-between the “chevrons”. Also note on the chat icons on the timeline, which show when chats happened and that there can be multiple, interspersed within the topic/regular replies. Also keep in mind these are just “quick” mockups (in the sense of basic, though they actually took me longer than I’d like to admit to create 
).
These are all great questions and potential answers. Ultimately I think this is something that will need to be figured out regardless of how chat and forum work together, and may also be more individual for each person or community. I suspect each community in particular will develop sort of cultural values/conventions for how they tend to handle that kind of thing, just as is the case with other conventions that are not enforced through settings or controls but are more about “how we do things here”.