The 'check continuously' is the thing.
I'm going to run it and have a look in a bit and get back to you.
It looks like 1st Word on the Atari ST also have a continuous spell checker.
https://en.wikipedia.org/wiki/1st_Word
From the link for 1st Word :
"Among the many new features was a spell checker with a 40,000 word dictionary, although lacking many American English terms,[11] a mail merge program, footnotes and semi-automated hyphenation.[12] The spell checker included the relatively rare, for the time, option to check on-the-fly. It also added document statistics display, including the number of characters, pages, etc"
Honestly I'd guess it's one of those things that possibly originated at Xerox Parc and then got added to consumer products from the 1980s onwards.
Personally, I remember it because I remember seeing Word 6 and thinking 'at last they have caught up to Prowrite'.
1st Word on the Atari ST 520+ with monochrome monitor: that was how the far future felt like in 1986.
With the exception of the somewhat wobbly cheap keyboard, that was the best and most distraction-free setup I have ever seen for WYSIWYG word processing (sadly never tried the Xerox workstations).
Sorry, but as a former Amiga user I have to jump on the opportunity to disrespect the Atari ST: wasn't the same thing (WYSIWYG text editing or even desktop publishing on a monochrome monitor) already possible years before with the Apple Mac? I mean, it was nicknamed the "Jackintosh" for a reason. Ok, it was only one year (the Mac being famously released in 1984, and the Atari ST in 1985). And of course the Atari ST was more affordable, it had that going for it.
MacWrite was very basic, even compared to 1st Word, although it did support multiple fonts, which weren't possible on the ST without some effort.
1st Word Plus (1987) was a huge improvement and used professionally in magazine publishing.
MS Word was always bloated and poorly-designed in comparison.
In terms of getting useful work done with a minimum of effort, all of the 80s WPs, both command-line and WYSIWYG, were superior to Word.