bug#42330: 26.3; Emacs does not change fonts per mode when changing buffers

bug#42330: 26.3; Emacs does not change fonts per mode when changing buffers

[Ray Dillinger]

Font changes set by mode hooks change when opening files but do not
change when switching buffers.  Font changes set by mode hooks can be
changed if the mode hook for a buffer is explicitly called, regardless
of whether the buffer is already in that mode.

When coding I prefer a 9-point monospace font. Accordingly my defaults
as set by Custom:

(custom-set-faces
 '(default ((t (:family "Latin Modern Mono" :foundry "unknown" :slant
normal :weight bold :height 90 :width condensed)))))

But recently I have been writing for publication, and I want to use the
publication font for text. Therefore I added a text hook:

(add-hook 'text-mode-hook
          (lambda()
            (setq fill-column 9999) ; each "line" (as defined by ending
in \n) is a paragraph.
            '(buffer-face-set "xft:-PfEd-DejaVu Serif-normal-normal-
semicondensed-*-21-*-*-*-*-0-iso10646-1 (#x4F)")
            (set-frame-font "DejaVuSerifCondensed 16" t t)
            (setq flyspell-mode t)
            ))

But then, after opening any text file, even though other mode-
appropriate changes would take place on opening/switching to a non-text 
buffer, emacs displayed ALL files with the text font.  I added a c-
mode-hook to explicitly undo the font changes for c files.

(add-hook 'c-mode-hook
          (lambda()
            (setq fill-column 160)
            (setq flyspell-mode nil)
            (set-frame-font "Latin Modern Mono Bold Condensed 9" t t)
            ))

Now if I open a text file it switches to the text font, and if I open a
c file it switches to the c font.  And that's correct behavior.

But if both files are already open, and I'm just switching between the
buffers, all the other c-mode or text-mode changes take place, but the
font does not change.

In order to get the font to change correctly, I have to run the hooks
explicitly: "M-x c-mode" seems like a silly thing to have to type if
already in C mode, but that's what it takes.  Likewise  "M-x text-mode" 
in the other case.

Something about the context of file opening allows font changes in the
mode hooks to succeed, where the context of buffer switching somehow
inhibits them.



In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.20)
 of 2020-05-16, modified by Debian built on x86-csail-01
Windowing system distributor 'The X.Org Foundation', version
11.0.12008000
System Description:     Debian GNU/Linux bullseye/sid

Recent messages:
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Configured using:
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files text-properties overlay sha1 md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 114727 11004)
 (symbols 48 22229 1)
 (miscs 40 72 291)
 (strings 32 33860 1728)
 (string-bytes 1 951529)
 (vectors 16 16148)
 (vector-slots 8 529561 8210)
 (floats 8 52 351)
 (intervals 56 514 125)
 (buffers 992 12))

bug#42330: 26.3; Emacs does not change fonts per mode when changing buffers

[Eli Zaretskii]

> From: Ray Dillinger <bear@sonic.net>
> Date: Sat, 11 Jul 2020 11:35:32 -0700
> 
> (add-hook 'text-mode-hook
>           (lambda()
>             (setq fill-column 9999) ; each "line" (as defined by ending
> in \n) is a paragraph.
>             '(buffer-face-set "xft:-PfEd-DejaVu Serif-normal-normal-
> semicondensed-*-21-*-*-*-*-0-iso10646-1 (#x4F)")

I believe the call to buffer-face-set above is in error, and thus
doesn't do what you want.

>             (set-frame-font "DejaVuSerifCondensed 16" t t)

And this switches the entire frame to the new font: again, not what
you want.

If you fix these two problems, i.e. use buffer-face-set correctly and
don't use set-frame-font at all, your setup will work as you want.

IOW, I don't think there's an Emacs bug here.

bug#42330: 26.3; Emacs does not change fonts per mode when changing buffers

[Eli Zaretskii]

> From: Ray Dillinger <bear@sonic.net>
> Date: Tue, 14 Jul 2020 18:17:07 -0700
> 
> On Sun, 2020-07-12 at 17:07 +0300, Eli Zaretskii wrote:
> > > 
> > If you fix these two problems, i.e. use buffer-face-set correctly and
> > don't use set-frame-font at all, your setup will work as you want.
> > 
> > IOW, I don't think there's an Emacs bug here.
> 
> You are correct.  I find myself floundering in the help subsystem and
> documentation, unable to find what I want or, often, correctly
> interpret what I find.  
> 
> Sorry to clog up your bug reporting system.  I got about half of the
> functionality I wanted, and interpreted the documentation I found in a
> way that made it seem that I ought to be getting all of it.  Of course
> there was other documentation that I had skipped ten times or more
> because some higher level of the documentation hierarchy was named
> something that I didn't think would be at all related to what I wanted
> to do.  
> 
> But I have managed to get the configuration to do what I want, finally.
> 
> 				Bear
> 
> Here are the relevant bits.
> -----clip-----
> 
> (defun fixed-face-buffer ()
>   "use a small fixed-width font for this buffer."
>   (interactive)
>   (setq buffer-face-mode-face '(:family "Latin Modern Mono" :foundry
> "unknown" :slant normal :weight light :height 90 :width condensed))
>   (buffer-face-mode))
> 
> (defun prop-face-buffer ()
>   "use a large variable-width font for this buffer."
>   (interactive)
>   (setq buffer-face-mode-face '(:family "DejaVu Serif" :height 160
> :width semi-condensed))
>   (buffer-face-mode))
> 
> 
> (add-hook 'text-mode-hook
>           (lambda()
>             (prop-face-buffer)
>             (visual-line-mode t)
>             (setq fill-column 9999) ; each "line" (as defined by ending
> in \n) is a paragraph.
>             (setq flyspell-mode t)
>             ))
> 
> (add-hook 'prog-mode-hook
>           (lambda()
>             (fixed-face-buffer)
>             (visual-line-mode nil)
>             (setq fill-column 160)
>             (setq flyspell-mode nil)
>             ))

Thanks, I'm therefore closing the bug.