* No support yet for Unicode graphemes spanning multiple codepoints.
* No support yet for right-to-left languages.
* Undo/redo may be sluggish in editor windows containing large files. Large
  files may grow sluggish in other ways.
* If you kill the process, say by force-quitting because things things get
  sluggish, you can lose data.

* Long wrapping lines can't yet distinguish between the cursor at end of one
  screen line and start of the next, so clicking the mouse to position the
  cursor can very occasionally do the wrong thing.
* Can't scroll while selecting text with mouse.
* No scrollbars yet. That stuff is hard.