Emacs

project.el

Emacs modes like eglot, the first-party LSP integration, rely on project.el to discover workspace roots. project.el's heuristics include checks for .git, which will fail in jj repositories that are not colocated. This can be addressed by adding ".jj" to project-vc-extra-root-markers, e.g.:

(use-package project
  :config
  (add-to-list 'project-vc-extra-root-markers ".jj"))

ediff as a merge tool

.jjconfig.toml

[merge-tools.ediff]
program = 'sh'
merge-args = ['-c',
  'emacsclient -c --eval "(ediff-merge-files-with-ancestor \"$0\" \"$1\" \"$2\" nil \"$3\")"',
  '$left', '$right', '$base', '$output']