The author has been working on Replit's IDE for six years and has focused on the code editor portion. The team initially used Ace but switched to Monaco due to its modern UI and features, but ultimately decided it wasn't suitable for mobile devices. They then adopted CodeMirror 6, which they found to be better suited for mobile and offering more extensibility and modularity. The author concludes that CodeMirror is now the preferred choice for Replit's code editor, with a strong community and documentation supporting its development.