I'm pretty sure that I was the one that came with the idea for adding them to generic text widgets when I implemented that in KDE (originally to add spell checking in KMail, since spell checking in email clients wasn't a thing yet). At that time spell checking was only done in word processors. Pretty quickly every other environment, including Windows and macOS followed suite.
> I'm pretty sure that I was the one that came with the idea for adding them to generic text widgets when I implemented that in KDE (originally to add spell checking in KMail, since spell checking in email clients wasn't a thing yet).
When was it that you added it to KMail?
Word '95 introduced the famous red squiggly underlines in 1995.
I believe KMail introduced spell-checking in 2004.
Reread the claim, the idea to add them to generic text widgets. I think it was around the mid 2000s when Firefox also started adding spellcheck, which is around the time I started learning how to spell things correctly.