Double-shift keybind is single-shift on some pages

**Describe the bug**
I'm trying to edit a page on the Warframe wiki, and all the markup is making Harper throw tons of errors. I can't type any markup myself tho, because every time I press `shift` it pops up the harper overlay and swallows my shift.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://wiki.warframe.com/ (or likely any other https://weirdgloop.org/ wiki)
2. Create an account (no email required) and edit a page.
3. Try to type special characters in the editor
4. Notice that Harper pops up the overlay on the first shift instead of the second.

**Screenshots**
Can't really screenshot this kind of behaviour.

**Platform**
Browser

**Additional context**
I've tried [wikipedia.org sandbox](https://en.wikipedia.org/wiki/Wikipedia:Sandbox) editor or the [mediawiki sandbox](https://www.mediawiki.org/wiki/Project:Sandbox) and neither have this problem.

The issue names no files or tests. Reproduce the behavior while editing wiki.warframe.com and compare it with the Wikipedia and MediaWiki sandboxes, then trace the overlay's Shift shortcut handling. Done means the overlay opens only on the second Shift and special-character markup can be entered normally.

Written by the indexing model from the issue text.