Pholio Mocks allow to post comments in a specific area of the image. However, users can't reply to these comments inline. It is possible to reply in the general comments area, but this breaks the conversation.
In comparison, Differential allows to post inline comments and their replies in a same place, in addition to the general comments area.
Our Design team thinks that inline conversations in mocks would be useful. See their originalk request at https://phabricator.wikimedia.org/T78022