It's been a while since I read this, but IIRC this article describes that bugs like these are exactly the reason why CodeMirror doesn't rely on contentEditable:

It's been a while since I read this, but IIRC this article describes that bugs like these are exactly the reason why CodeMirror doesn't rely on contentEditable: https://codemirror.net/doc/internals.html https://codemirror.net/doc/internals.html > CodeMirror 1 was heavily reliant on designMode or contentEditable (depending on the browser). Neither of these are well specified (HTML5 tries to specify their basics), and, more importantly, they tend to be one of the more obscure and buggy areas of browser functionality—CodeMirror, by using this functionality in a non-typical way, was constantly running up against browser bugs.