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)

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index a86c12a0db7ad30cd92211c56f64ed00a515367d..7ec843180b84a3b129e3c8ec3dec1931a2b2ba7d 100644 (file)
--- 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 3cc5f10582a9088b271d85ff2a93034159f6d223..7037c8c943a21a33b2bb586c2f24179694b0a0af 100644 (file)
--- 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 7ad09de07654f572c8dee09abeee4cdb291db1c7..45bdca2f5a5eff4352060d5c7053989136a2473d 100644 (file)
--- 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