threaded comments: broken GUI design leads to replies in wrong hierarchy

~bug

summary

The broken GUI design for threaded comments leads to replies in the wrong hierarchy.

current behavior

user2 uses the normal commentbox instead of the discussionreplybox:

@user1: comment#1 (starting a discussion)
mentioning @user2
  |  |
  |  @user1: comment#3 in reply to comment#2
  |
@user2: comment#2 in reply to comment#1

expected behavior

@user1: comment#1 (starting a discussion)
mentioning @user2
     |
     @user2: comment#2 in reply to comment#1   
     |
     @user1: comment#3 in reply to comment#2

examples in the wild

• https://gitlab.com/gitlab-org/gitlab-ce/issues/30299#note_115993896
• https://gitlab.com/gitlab-org/gitlab-ce/issues/30299#note_117958390

Output of checks

This bug happens on GitLab.com