Not a big fan of the way Facebook started hiding their comments with "top" ro dropdiown options either to be honest, but maybe just a "View All Comments" button that expands all comments or launches the popup. I think a setting to allow or not allow truncation would be good as well, or to change the order of first in or last (I think this may already exist)

Maybe experiment with a scrollbox with standard height with an expand button to remove the scroll would work too so you wouldn't need to load the extra comments to see the natural flow of comments they would all be there jsut a smaller window of them till you expand.

Bottom line never made sense to remove random parts of the thread as none of the included comments flow together at all.