Apparently we can't use multiple of the same role of slot per page, so we can't just have each thread be a slot we add to the page. What this means is we need to either use a single slot that contains all the threads or get that changed in core.
In our case this is probably fine for the time being, as we'll only ever be plonking the threads at the end of a thing for the base use case, and it should simplify the display handling as well?