I've worked in 3 different WYSIWYG editors for web and desktop applications over the years, lightly contributed to a handful of other open-source editors, and spent plenty of time building my own personal editors from scratch (and am currently using gpt-5 to fix my own human bugs in a rewrite of the Notebook.ai text editor that I re-re-implemented ~8 years ago).

Editors are incredibly complex and require domain knowledge to guide agents toward the correct architecture and implementation (and away from the usual naive pitfalls), but in my experience the latest models reason about and implement features/changes just fine.