bug#50970: 29.0.50; Italics cause fill column indicator misalignment

[Eli Zaretskii]

> From: Carlos Pita <carlosjosepita@gmail.com>
> Date: Sat, 2 Oct 2021 15:02:08 -0300
> Cc: 50970@debbugs.gnu.org
> 
> Which font is that?  I cannot reproduce with the default font Emacs
> uses here.  Do you see the same results with all monospaced fonts,
> once you customize the comment face to use italics?
> 
> 
> Source Code Pro, but it doesn’t seem to be relevant.
> 
> I have been able to reproduce it with a minimal setup (see the
> attached screenshot):
> 
> (setq inhibit-startup-screen t)
> (setq fill-column 80)
> (global-display-fill-column-indicator-mode)
> (custom-set-faces
>  '(default ((t (:height 160))))
>  '(font-lock-comment-face ((t (:slant italic))))

Doesn't reproduce here with these settings.

> The problem is that it doesn’t always happen, I had to restart emacs
> many times (I’m doing `emacs .emacs.d/init.el` one time after another)
> until the issue happened. Due to the random nature of the issue it’s
> not possible to ascertain that the (:height 160) part is strictly
> necessary, but let’s say that I’ve rejected the null hypothesis that
> the issue happens at default font size, but maybe it’s that I’m
> shortsighted enough as to not perceive it at the smaller font size.

That's too bad, because it means I have no reliable recipe to
reproduce and debug the problem...

Does anyone else see this and can provide a reproducible recipe?