"harper-render-box" code being inserted into the text field of the forum posts

**Describe the bug**
After installing Harper in Firefox, I noticed a strange behavior in thebriarpatchforum.com forums.

Code starting with "harper-render-box" is being inserted directly into the text field of my forum posts. Even when I disable Harper for that particular site, a shorter string of code is till inserted. I have to edit the post to delete it.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to thebriarpatchforum.com forums. The text boxes have two tabs underneath: "Preview" and "BBCODE"
2. If I go to the BBCODE tab, the following lines of code ares inserted in the text box, and can be viewed in "Preview" as well. It also happens when I submit a post.

``

If I disable Harper for the forum, the following line is still inserted visibly in the text box:

``

3. Only by disabling Harper in Firefox eliminates this behavior.

**Expected behavior**
I assume that instead of that, the Harper popup would become active. (?)

**Screenshots**
DarkReader disabled, Harper disabled.
https://imgur.com/OI5h4HD

Both Harper and Dark Reader enabled.
https://imgur.com/XoWyyWc

Harper Enabled. Dark Reader disabled.
https://imgur.com/ktaKOI5

**Platform**
Firefox.

**Additional context**
For some reason, code mentioning Dark Reader is also visible. This only happened after Harper was installed.

I like the add-on, even if I have only used it for a few days. I haven't noticed this behavior elsewhere ATM.

-Ed

Reproduce the issue in Firefox on thebriarpatchforum.com forum, using the BBCODE and Preview tabs with Harper and Dark Reader enabled or disabled. Trace why the harper-render-box markup is inserted into the post field; done when the markup no longer appears in forum content and the Harper popup behaves as expected.

Written by the indexing model from the issue text.