It looks exactly like the separate conversation tabs used for chatting with friends on Newsfeed.
We all know, Facebook is constantly evolving, the enhancement of this pop-up tab might be useful if you're actually into commenting and replying, (THAT, I AM.. AND I AM ACTUALLY ENJOYING THIS ENHANCEMENT!) but if it is not the case, it probably appears to be annoying and demanding.
“We've heard from people that they would like an easier way to participate in conversations on a post while they are still in News Feed so we are rolling out a new option that opens up a window when someone comments on your post, replies to your comment or tags you in a comment."
To me it's an amazing update but if you don't find it any useful you can turn the feature off too.
How to do that? Simple, go to settings >> notifications >> Control your recent notifications >> Turn off. And your're done, tadaa!!