bug#55159: 29.0.50; ruler mode collides with header-line-format

[Eli Zaretskii <eliz@gnu.org>]

> From: Jean Louis <bugs@gnu.support>
> Date: Thu, 28 Apr 2022 07:56:48 +0300
> 
> 
> Following does not work as expected:
> 
> (progn
>   (ruler-mode)
>   (setq header-line-format "Instructions"))
> 
> And it should -- I am not sure, but I guess that it worked before,
> because I have a function that always uses ruler-mode and title.

In what version of Emacs was that?

> The header-line should be displayed above the ruler in my opinion. Both
> combinations should work regardless if header-line-format comes first or
> second. 

ruler-mode.el uses the header-line to display the ruler.  It always
did.  So by overwriting header-line-format in a buffer under
ruler-mode you shoot yourself in the foot; don't do that.

I very much doubt that it ever worked for you as you describe, because
ruler-mode.el used header-line since it was first introduced in Emacs
22.1.  From NEWS.22:

  ** The ruler-mode.el library provides a minor mode for displaying an
  "active" ruler in the header line.  You can use the mouse to visually
  change the `fill-column', `window-margins' and `tab-stop-list'
  settings.

So I see no bug here, but a cockpit error.