emacs-29 003759a6dca: Explain effect of variable-pitch fonts on fill-column

[Eli Zaretskii]

branch: emacs-29
commit 003759a6dca730df6fae6c9bc8d4a47761b10014
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Explain effect of variable-pitch fonts on fill-column
    
    * doc/emacs/text.texi (Fill Commands):
    * doc/emacs/display.texi (Displaying Boundaries):
    * lisp/display-fill-column-indicator.el
    (display-fill-column-indicator-mode): Document caveats of using
    variable-pitch fonts with 'fill-column' and its indicator.
    (Bug#61677)
---
 doc/emacs/display.texi                | 6 +++++-
 doc/emacs/text.texi                   | 7 ++++++-
 lisp/display-fill-column-indicator.el | 3 +++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index a86c12a0db7..7ec843180b8 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1402,7 +1402,11 @@ right-to-left paragraphs.
 functionality especially in @code{prog-mode} and its descendants
 (@pxref{Major Modes}) to indicate the position of a specific column
 that has some special meaning for formatting the source code of a
-program.
+program.  This assumes the buffer uses a fixed-pitch font, where all
+the characters (with the possible exception of double-width
+characters) have the same width on display.  If the buffer uses
+variable-pitch fonts, the fill-column indicators on different lines
+might appear unaligned.
 
   To activate the fill-column indication display, use the minor modes
 @code{display-fill-@-column-indicator-mode} and
diff --git a/doc/emacs/text.texi b/doc/emacs/text.texi
index 3cc5f10582a..7037c8c943a 100644
--- a/doc/emacs/text.texi
+++ b/doc/emacs/text.texi
@@ -619,7 +619,12 @@ variable @code{fill-column}.  The default value 
(@pxref{Locals}) is
 is to use the command @kbd{C-x f} (@code{set-fill-column}).  With a
 numeric argument, it uses that as the new fill column.  With just
 @kbd{C-u} as argument, it sets @code{fill-column} to the current
-horizontal position of point.
+horizontal position of point.  Note that, by its very nature,
+@code{fill-column} is measured in column units; the actual position of
+that column on a graphical display depends on the font being used.  In
+particular, using variable-pitch fonts will cause the
+@code{fill-column} occupy different horizontal positions on display in
+different lines.
 
 @cindex centering
 @findex center-line
diff --git a/lisp/display-fill-column-indicator.el 
b/lisp/display-fill-column-indicator.el
index 7ad09de0765..45bdca2f5a5 100644
--- a/lisp/display-fill-column-indicator.el
+++ b/lisp/display-fill-column-indicator.el
@@ -53,6 +53,9 @@ customize `display-fill-column-indicator-column'.  You can 
change the
 character for the indicator setting `display-fill-column-indicator-character'.
 The globalized version is `global-display-fill-column-indicator-mode',
 which see.
+This minor mode assumes the buffer uses a fixed-pitch font; if you
+use variable-pitch fonts, the indicators on different lines might
+not appear aligned.
 See Info node `Displaying Boundaries' for details."
   :lighter nil
   (if display-fill-column-indicator-mode