bug#61196: 30.0.50; X11 rendering of horizontal line in log-edit mode fails with some fonts

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Dima Kogan <dima@secretsauce.net> writes:

> Hi. I'm using a bleeding-edge build of emacs from git, but this
> particular issue I've been seeing for maybe 6 months now, or maybe even
> a year.
>
> I'm using a GTK build of Emacs running on X11 on GNU/Linux. Some of that
> is significant.
>
> I can tickle the bug by selecting a particular font, entering
> log-edit-mode, and entering some text:
>
>   rm /tmp/xxx;
>
>   emacs -Q -geometry 40x8 \
>     --eval "(progn (setf (alist-get 'font default-frame-alist) \"-adobe-courier-medium-r-*-*-12-*-*-*-*-*-iso8859-*\")
>       (require 'log-edit)
>       (find-file \"/tmp/xxx\")
>       (log-edit-mode)
>       (insert \"a:b\\n\\nc d\"))"
>
> The result looks like the attached 12.png. Note the unrendered 000A
> character block. This is a bug. It should be rendering a horizontal line
> instead. And if I select a different font, it does work. I can do that
> by changing the "-12-" in the above command to "-22-". The result is
> attached in 22.png.
>
> I see it in the wild all the time when making version control commits in
> emacs, and typing the commit message.
>
> Thanks

When I last investigated this problem (I use the X core font backend all
the time), it was that X fonts typically have no glyph for the newline
character, which Emacs seems to expect.