IMO: Just do a normal site where links take you to pages. Don't be creative with the UI. Maybe you could use some bog-standard wiki software?
The content itself looks very valuable, but because the UI is so bonkers, it's just too hard to use or link to.
It actually used to be normal HTML... the site is the first wiki. For some reason it was reimplemented as a broken JS app.
https://www.wired.com/2012/07/wiki-inventor/
But his implementation of it for some reason involved rewriting it as an SPA, which he entirely botched.
Adding my opinion, also let the text use the page width. Limiting it to 120 characters or whatever is fine, but what this site does is really not.
On mobile I just tap outside of the panel to close it. Probably on desktop it will close as well if you click outside of it?
Update - took me a solid 3 minutes of futzing with it to figure out you can close something by clicking on it's parent. Clicking on the grey area does not work.