Subject: Re: bug#73862: [PATCH] Add `header-line-active` and `header-line-inactive` faces.

From: Aaron Jensen <[email-masked]>
Date: Thu, 5 Dec 2024 08:02:06 -0800

On Wed, Dec 04, 2024 at 11:51 PM, Eli Zaretskii <[email-masked]> wrote:

> Once again, please show some simple Lisp to reproduce the phenomena you
> are observing. It is hard to discuss these highly technical issues on this
> abstract level.
>

Happy to. Sorry, I didn't think it was necessary given Eshel's providing of
the recipe. It wasn't lisp though, so here's the lisp version from emacs -Q:

   1. (setq header-line-format "Some Header")
   2. (face-remap-set-base 'header-line 'highlight)
   3. (set-face-attribute 'default nil :height (+ (face-attribute 'default
   :height) 10))
   4. (switch-to-buffer-other-window "new")
   5. From within new buffer/window: (set-face-attribute 'default nil
   :height (+ (face-attribute 'default :height) 10))

You'll see after step 3 the header line switches to the highlight face.
After step 5, the header line in the original buffer switches away from the
highlight face, which is unexpected.

If you then switch back to the original buffer and change the font size
again, the highlight will display again. Switching back to the new buffer
and changing the font size causes it to disappear again, so I was mistaken
about it only happening once. It just so happens that a full redisplay only
triggers once for me in my normal usage when I have a window without the
header line override specified.

On the very basic level of the display code, when a display iterator is
> initialized, the window for whose display the iterator is used must be
> given to init_iterator as its argument, and the buffer of that window must
> be temporarily made to be the current buffer. So this cannot be the problem
> in this case. If init_iterator would be passed an incorrect window, we'd
> have much more grave display problems than this minor issue.
>

Understood, but that's not what I was attempting to describe. I was trying
to say that the window passed to init_interator IS the correct window. And
that that is used to resolve the remap for the inactive/active faces.
However, it does not appear that that same window is used to resolve
remapping of faces that are inherited by the inactive/active faces.

>From what I can tell, this only applies to header-line-active and
header-line-inactive (and their mode line equivalents) and it does not
apply to other faces in the header line that may inherit from remapped
faces, so that's good.

Also, I understand what you're saying that remapping mode-line was decided
in Emacs 29 to not be supported and the conclusion may be the same for
header-line. If that's what it needs to be, that's fine. As Eshel said, it
will be a breaking change because people have relied on this behavior.

Thanks,