I really wanted to like it, but code starting with "harper-render-box" is being inserted directly into the text field in a forum I visit regularly. Even when I disable Harper on that particular site, a shorter string of code is still inserted. I have to edit the post to delete it. I do not know WHY it happens, only that it also seem to affect my installation of Darkreader, since some of its injected code is revealed as well. Only uninstalling the addon completely cleared the issue.

I posted the issue on GotHub as clearly as I could: githubDOTcom/Automattic/harper/issues/2913. I will try it again after several months have passed, to see if the issue is gone.