We've been on CodeMirror 3.23, and attempted an upgrade to 4.2 in the

past. That upgrade resulted in infinite loops and display problems, so

we abandoned it. It seems though that CodeMirror 5.5 is pretty stable

for our needs, and has the added benefit of mobile support, so we're

switching over as a trial run.
This was also build with additional syntax highlighter modes for

CoffeeScript, CSS, Go, HTML, Perl, PHP, Python, ReStructuredText, Ruby,

shell scripts, SQL, XML, and YAML. This will help with providing a

better preview of code when writing using Markdown triple-backticks.
Along with these additions, I've turned off the elasticChars switch,

which attempted to match up braces to the correct indentation level. This

wasn't working in practice for us, for some reason, and it's best not to be

too much like an IDE when people are expecting a typical text field.

Tested this in Chrome, Firefox, and iOS. I didn't see any of the

lockups or anything we had before (which may be due to fixes in

CodeMirror, or fixes in our event handling, which may have previously

triggered infinite loops).
I tested each of the new syntax modes, and they seem to work well.