bug#21848: Bug in emacs: filling a certain region with (fill-region) causes a certain character to disapear

[Glenn Morris]

Ruy Exel wrote:

> 2) Mark a region consisting of the first two lines, namely
>
> | a |
> | a
>
> 3) Observe that these two lines contain *three* vertical bars
> 4) Run the command "ESC-x fill-region"
> 5) Observe that thare are only *two* vertical bars now!


This is not a bug, but rather "adaptive filling" behaving as designed.
Compare the result if you first: (setq adaptive-fill-mode nil).