I found the solution on this thread. It turned out that since 2011,
vimrc_example.vim was updated to include this
textwidth=78 rule as an autocommand, and my
vimrc_example.vim (probably because I based it on the original — which means this situation will apply for many users).
One solution is to find the line in your
.vimrc, that says
And after that line, insert the following line to unset the particular autocommand that's making a nuisance:
au! vimrcEx FileType text
How I found the solution: I suspected that it was an autocommand, so I tried looking through all autocommands by using the
:autocmd command. But there were just too many. To simplify things, I inserted that output into a spare buffer, as described here. Searching for
tw=78 showed the culprit autocommand with the keyword
vimrcEx, and then asking Google for the keyword (or I could have grepped Vim's directory) lead us to the solution.