As i seen, this is incompatible with basical EditTools functionality (without enhancing by JavaScript). This is incompatible with Extension:Charinsert, or by another cause? In my opinion, this is should be described...
Topic on Extension talk:CodeMirror
Hi Kaganer, sorry you're having trouble with the highlighting. Can you tell me what browser you're using? We've currently got a bug report about specific browsers, and I want to know whether you're in the same situation.
Yandex Browser (based on Chromium) This also occured in the Microsoft Edge.
Okay, thanks for the report. I've added it on one of our work tickets -- phab:T179287 -- and we'll look into it.