Toggling a diff discussion does not load the comment draft

Summary

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17011:

Toggling a diff discussion in a merge request only displays the comment draft after full page reload.

Steps to reproduce

• open merge request
• create new discussion
• start writing a comment (but don't submit)
• toggle discussion to hide it
• toggle discussion to make it visible

What is the current bug behavior?

comment draft stays invisible until page reload:

invisible-draft

What is the expected correct behavior?

comment draft is visible after toggling the discussion