Very few features on purpose. Every editor I tested had all the features I added and more.

It is worth noting that KTextEditor is a fully-featured library. Like, line numbers+gutter (for eventual git status icons), undo/redo, save, warn on exit for unsaved changes, syntax highlighting, color theming. It does 95% of what we'd all call "editing". But it doesn't do things like tab interface, project explorer, terminal pane, output windows, etc.

What the library didn't have were LSP features, of which I only implemented a few (error squiggles under things that fail the type check, go to definition). Notable absent are completions and hover features for things like help. I also only added (and tested) LSP servers for typescript, Rust and Go.

My plan has been: do as little as possible until I need something, then ask Fable to add it.

edit: I guess one thing I added I didn't see everywhere else was a built-in Markdown preview. But many editors have that (VSCode definitely does) so it isn't special.