Acrobat (years ago) had this ability, I believe.

Currently, we can expand/collapse all comments.
We can also expand/collapse comments for an entire page.

Please add an option at the comment level that would expand/collapse the replies to a specific comment.

When a page has 30 markups, and each markup has 5 replies (yes, this has happened :-)), there are now 150 comments showing in the comment list for just that single page.

Being able to collapse a comment thread would be a massive improvement in usability of the commenting feature, I believe.