This is fixed as of version 1.0.94 going live right now.
Per the release notes:
The "Reply" button in the action bar when selecting a comment is now usable while already writing a comment
When the commenting UI is visible, the post which you are commenting on doesn't show the "Add Comment" button, since you're already commenting on it. You can see this in the second screenshot in the question vs the first one.
The issue ended up being that we used the same
can_add_comment variable to control showing both that "Add Comment" button, and the Reply button. When that commenting UI is open, to hide the "Add Comment" button we set
can_add_comment to false, which then caused the Reply button to disappear too.
As of version 1.0.94, we use a
can_add_comment variable and a
is_mid_commenting variable, which lets us hide the "Add Comment" button separately from showing/hiding the Reply button.
It's not perfect, because you can keep on hitting Reply on the same comment over and over again and each time it'll add the
@Username to the front of your pending comment, but it makes a whole lot more sense than what was happening before.