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Re: Fill column indicator functionality

From: Ergus
Subject: Re: Fill column indicator functionality
Date: Fri, 5 Apr 2019 23:15:43 +0200
User-agent: NeoMutt/20180716

On Fri, Apr 05, 2019 at 10:03:07PM +0300, Eli Zaretskii wrote:
Date: Fri, 5 Apr 2019 20:11:33 +0200
From: Ergus <address@hidden>
Cc: address@hidden

>  (if (eq (aref (car (internal-char-font nil ?│)) 0)
>          w(face-font 'default))
>       ;; can use the default font for U+2502
>(But we should use the actual value of
>display-fill-column-indicator-char instead of the literal character.)

Actually this is what we have now:

(define-minor-mode display-fill-column-indicator-mode
  "Toggle display fill column indicator.
This uses `display-fill-column-indicator' internally.

To change the position of the column displayed by default,
customize `display-fill-column-indicator-column' you can change the
character for the indicator setting `display-fill-column-indicator-character'."
  :lighter nil
  (if display-fill-column-indicator-mode
        (setq display-fill-column-indicator t)
        (unless display-fill-column-indicator-character
          (if (char-displayable-p ?\u2502)
              (setq display-fill-column-indicator-character ?\u2502)
            (setq display-fill-column-indicator-character ?|))))
    (setq display-fill-column-indicator nil)))

I think we only need to extend the inner if with your condition, but I
got an error because (internal-char-font nil ?│) always return nil in a
terminal and aref has issues with that.

For terminal frames, char-displayable-p is okay and should be left
alone as the only test there.  The test I proposed is only needed for
GUI frames.

In a gui I get this error:

Debugger entered--Lisp error: (wrong-type-argument arrayp #<font-object 
"-1ASC-Liberation Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1">)
  aref(#<font-object "-1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1"> 0)
  eval((aref (car (internal-char-font nil 9474)) 0) nil)

Oops, sorry, I copied the wrong expression.  This is the correct one:

 (aref (query-font (car (internal-char-font nil ?│))) 0)

Btw, there could be a problem when the mode is turned on on a GUI
frame, where '│' can be displayed OK, and then a TTY frame is created
in the same session, where that character cannot be displayed.  I
wonder how to deal with this issue.

There is also the problem that the user could activate
display-fill-column-mode and then change the font.

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