CodeMirror 6 is a new code editor library for the web, a from-scratch implementation based on the experience of building and maintaining versions 1 to 5 for the past 13 years. It aims to be more extensible and accessible than previous versions.
As of today, version 6.0 is stable. Going forward, probably at least several years, all new releases will be under the 6 major version, and be backwards compatible.
The library has been usable and largely stable for over a year, with only minor breaking changes. I generally prefer to release late, to avoid having too many regrettable mistakes slip into the stable release, which would then have to be kept there indefinitely. Without a doubt there will be things that, a year from now, I wish I had released differently, but by having users work with the code in production for a significant amount of time, a lot of small issues and sources of friction have been found and resolved before being set down in stone.
Work on this system started four years ago, with Prototype Fund funding the initial work. It was announced publicly and crowd-funded a year after that, built out into a useable system in the two years after that, and refined and stabilized in the past year. During the first two years, I collaborated with Adrian Heine on the design and implementation of the initial system.
For some more background on the new library, see the blog posts on Lezer (the parser system), facets (the extension system), and collaborative editing. For an overview of the entire system, take a look at the system guide. For a rough summary of what changed in the interface since 5.x, see the migration guide.