The best alternatives to CoderMirror are Screenful for Trello, CodeTime, and Mastery. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of CoderMirror?
Tanka— Tanka = Slack + Glean, built for fast-growing startups.
CodeTime is a way for you to understand how you spend your time coding. CodeTime will automatically track the amount of time you spend on various languages as well as keep a convenient timer in your status bar to see all the time you've spent coding!