macOS built-in spell checker performs badly - what can I do?
There is a small application that runs in the background called popclip which has add-ons to check your spelling. Seems to work almost everywhere. It has many other add-ons to help you work. I've been using pop=clip for years with no problem. The spell checker seems to run everywhere and if you select the misspelled word and double-click it, popclip presents you with an alternative spelling.